Rigid connecting elements of bridge prostheses. There are 3 types of rigid connections:
Cast.
Conventional or laser welding.
Ceramic.
cast connections are prefabricated from wax on wax templates of artificial teeth and retainers, so that the bridge can be cast as a single block. This eliminates the need for further welding. But the casting should be more accurate, the more units the prosthesis includes. Small deformations that occur when the molten metal is cooled may be quite acceptable in the manufacture of one unit, but when multiplied many times, lead to an unsatisfactory final result.
cast connections stronger than welding, in addition, they are easier to hide. For this reason, long bridges are often cast in 3-4-unit pieces, with the dividing line passing through the artificial tooth. The framework of an artificial tooth before ceramic veneer is restored by high-precision welding - thus, all joints are cast. The welding of an artificial tooth is very strong, firstly, because of the larger area compared to the connecting element, and secondly, because of the ceramic coating.
An increasingly popular way to connect bridge components becomes the technique of laser welding. It is stronger than usual, as well as simpler and faster, although it requires complex and expensive equipment.
Connections using conventional and laser welding is used if the components of the bridge are made separately. This may be necessary when they consist of different materials (for example, a fixing crown made of gold and a ceramic-metal artificial tooth).
Ceramic compounds used only in all-ceramic prostheses. It is beyond the scope of this book to describe how they are made, but the principle of accessibility for hygienic measures should also apply to such compounds.
Movable connecting elements. Movable connecting elements are always designed so that the artificial tooth does not fall under the action of chewing load. This means that the recess of the smaller retainer must always have a solid base against which the protruding part of the connection would abut. Sometimes, with small artificial teeth and a short denture, this is the only force that needs to be resisted, and the recess in the retainer can be quite shallow. This is the most common design for fixed-retained prostheses that require minimal preparation.
However, with a longer arm prosthesis the movable joint must also resist the lateral displacement moment acting on the artificial teeth, and (if the movable joint is located mesially) the forces directed distally and facilitating the separation of the parts of the prosthesis. In this case, the connection groove should be in the shape of a pigeon's tail and taper so that the pin can move slightly up and down in it and at the same time firmly rest against the base.
There are several manufacturing methods. A smaller recessed retainer can be waxed first, then cast and finished with a conical bur. After that, a layer of wax is manually applied to the artificial tooth so that it matches the obtained shape of the recess, casting is performed according to the wax pattern. Before fitting the frame, both parts are interconnected.
In some cases excavation can be made on a ready-made cast frame, which is then placed in the oral cavity, after which casts are taken, including prepared abutment teeth.
Can be used acrylic templates embedded in the wax model of an artificial tooth and a smaller retainer. The smaller retainer and the rest of the prosthesis are then cast separately.
As movable connecting elements ready-made metal pin-groove fasteners are also used, but they provide too rigid a grip, due to which the mobility of the parts of the prosthesis can be sharply limited. In this case, the smaller retainer should have a higher than usual degree of retention to the abutment.
Ready-made screw fasteners used as part of fixed bridges to connect 2 parts in case the abutment teeth are not parallel.
- Return to the section heading " "
Due to the limited size of the tree, the creation of building structures of large spans or heights from it is impossible without connecting individual elements. Connections of wooden elements to increase the cross section of the structure are called rallying, and to increase their longitudinal length - splicing, at an angle and attaching to the supports - anchoring.
By the nature of the work, all the main connections are divided into:
Without special connections (frontal stops, cuts);
With compressive bonds (shoe keys);
With bonds working in bending (bolts, rods, nails, screws, plates);
With tensile bonds (bolts, screws, clamps);
With shear-shear bonds (adhesive seams).
According to the nature of the work of the joints of wooden structures, they are divided into pliable and rigid. Compliant are made without the use of adhesives. Deformations in them are formed as a result of leaks.
Connections of elements of wooden structures according to the method of transferring forces are divided into the following types:
1) joints in which forces are transmitted by direct contact of the contact surfaces of the elements to be connected, for example, by adjoining in the supporting parts of the elements, notching, etc.;
2) connections on mechanical links;
3) joints on adhesives.
Mechanical connections in wooden structures are called working connections of various types made of hardwood, steel, various alloys or plastics, which can be inserted, cut, screwed or pressed into the wood body of the connected elements. The most widely used mechanical ties in modern timber structures are dowels, dowels, capercaillie bolts, nails, screws, keyed washers, dowel plates, and metal toothed plates.
The bearing capacity and deformability of wooden structures depends to a greater extent on the method of connecting their individual elements. Connections of tensioned wooden elements are usually associated with their local weakening. In the weakened section of the stretched wooden elements, there is a concentration of dangerous local stresses that are not taken into account by the calculation. The greatest danger in butt and nodal joints of tensioned wooden elements are shear and splitting stresses. It is exacerbated when these stresses are superimposed on the stresses that arise in the wood due to its shrinkage.
Chipping and tearing along and across the fibers are among the brittle types of wood work. In contrast to the work of structural steel in wood, plastic stress equalization does not occur in these cases. In order to reduce the risk of consistent, in parts, brittle fracture from chipping or rupture in the stretched elements of wooden structures, it is necessary to neutralize the natural fragility of wood by the viscous compliance of their joints. The most viscous types of wood work, characterized by the greatest amount of work of strong resistance, include crushing. In other words, the requirement of toughness for joints of all types of elements of wooden structures is reduced to the requirement to ensure the equalization of stresses in parallel beams or boards, using the viscous ductility of the wood to collapse before brittle fracture from rupture or chipping could occur.
To impart viscosity to the connections of tensioned wooden elements, as a rule, the principle of fractionation is used, which allows avoiding the danger of chipping wood by increasing the chipping area (draw a connection with one bolt and with several smaller diameters).
Contact connections of wooden elements. Front notch.
Contact connections of wooden elements mean connections in which forces from one element to another are transmitted through their processed and sawn contact surfaces. Additional working links supplied in such connections carry the function of fixing individual elements and serve as emergency links. With contact joints, the work of wood on crushing is decisive. The advantage of a connection with a simple support is a slight effect on their work of wood deformations during fluctuations in temperature and humidity conditions, especially if the compressive forces of the connected elements are directed along the fibers. Contact connections with compression perpendicular to the fibers are found in the joints of racks at the junction with horizontal crossbars, supports of girders, beams, trusses on walls. In these cases, the calculation is reduced to determining the verification of the crushing stresses on the contact surfaces and comparing them with the calculated resistance. The resistance of wood across the fibers is small, then under the action of large forces it is necessary to increase the supporting areas or contact surfaces of the elements to be joined. The methods are shown in the figure.
In the absence of the possibility of increasing the contact area, pads are used on the sides of plywood on dowels or glue, which distribute the load to a greater depth of the element. Another method of reinforcing glued beams in the bearing part, developed in our country, is sawing the bearing angle at an angle of 45º, turning it 90º and gluing it. This achieves the maximum resistance of wood to crushing (along the fibers).
Contact connections of wooden elements with the action of forces along the fibers are encountered when building racks along the length. In this case, the collapse resistance is maximum, but there is a danger of interpenetration of wooden elements due to the fact that the denser layers of one element may coincide with the less dense layers of another. To prevent displacement of the ends, cylindrical dowels are installed at the ends or side plates. In this case, the calculation for collapse is not carried out, being limited to the calculation for buckling.
The work of wood on crushing at an angle occurs when connecting inclined elements (see Fig. Upper truss belt). Check for crumpling at an angle.
Front notch. A notch is a connection in which the force of an element working in compression is transferred to another element directly without liners or working connections. The main area of application is nodal connections in block and log trusses, including in support nodes adjoining the compressed upper chord to the stretched lower one. The elements to be connected must be fastened with auxiliary connections - bolts, clamps, brackets, which are designed for mounting loads.
The front cut can lose its bearing capacity when one of 3 limit states is reached: 1) by crushing the stop platform, 2) by chipping the stop platform, 3) by breaking the lower chord weakened by the cut.
The crushing area is determined by the depth of the cut, which can be no more than 1/3 of the height of the tensioned element. Decisive, as a rule, is the bearing capacity of the cut from the shearing condition. According to SNiP II-25-80, the frontal cutting for shearing for an angle of 45º is calculated by determining the average shear stress along the length of the shearing area according to the formula: 
, where is the calculated resistance of wood to chipping, is the estimated length of the chipping area, e is the shoulder of the shear forces, -=0.25 coefficient. For an angle of 30º: .
Keyed connections and keyed washers.
Dowels are hardwood, steel, or plastic inserts that are placed between braced members to prevent movement. There are prismatic wooden longitudinal dowels, when the directions of the fibers of the wood of the dowels and the connected elements coincide, and transverse, when the direction of the fibers is perpendicular. Parallel keys work on crushing and chipping. It is possible to use metal tee keys. A distinctive feature of the keys is the appearance of an overturning moment and, as a result of this, the occurrence of a thrust between the connected elements. For the perception of thrust, it is necessary to install the coupling bolts. The length of the key is taken not less than . The depth of insertion of dowels into the beams should be taken at least 2 cm and not more than 1/5 of the height of the beam, and logs - at least 3 cm and not more than ¼ of the log diameter.
The calculation of connections on keys is reduced to checking the bearing capacity for crushing and chipping. When calculating in multi-row connections, a coefficient of 0.7 is introduced, due to the uneven distribution of forces.
To connect wooden structures at different angles, round center dowels with a tie bolt in the center are placed in the nodes.
Key type washers are the most widely used. Joints on toothed keys are characterized by high load-bearing capacity and toughness. They are pressed into the body of the wood by impact or special clamps. The disadvantages include: the formation of cracks in the mating elements, a decrease in the bearing capacity due to the uneven pressing of the keys in multi-row connections.
Connections on cylindrical pins (steel, oak, plastic, aluminum, nails, screws, capercaillie) and lamellar.
Nail joints with inserts in knots and on metal toothed (nail) plates.
Nail connections with inserts in knots
When large forces act in the nodes or several elements are connected, it is difficult to ensure the transmission of forces through the contact surfaces of all mating elements. In such cases, it is advisable to use various inserts in the form of nodal plates, which increase the area of the node and at the same time create a multiple shear of working connections. As nodal inserts, plates of steel and plywood are most often used. They can be located outside (linings) and attached from the outside to the wood of the connected elements using single-cut dowels or located inside the wooden element (lining) in special cuts so that the working connections can work as multi-cut dowels.
Connections with pads and gaskets on bolts or blind cylindrical dowels are allowed in cases where the necessary tightness of the dowels is ensured. Blind steel cylindrical dowels must have a depth of at least 5 dowel diameters. The transfer of forces from one wooden element to another occurs sequentially through the dowels, plate and dowels of another wooden element. The cross section of the plates is assigned from the condition of calculating the tension along the weakened section and ensuring the crushing strength in the nest under the dowel. In dowel joints, steel plates with a thickness of at least 5 mm are usually used. Nest holes for dowels are usually drilled simultaneously in a tree and in a plate. In this case, if the gaskets are steel, the first time a hole is made with a drill with d corresponding to the socket of the dowel in the wooden element (0.2–0.5 mm less than d of the dowel), then the metal plate is removed from the cut and the holes in it are reamed to the size of the dowel diameter.
The manufacturing technology of these joints is relatively laborious, but justified by the fact that when placing metal elements inside wood (the ends of the dowel and bolts are left 2 cm below the surface of the element and glued on top with a wooden insert), the fire resistance of wooden structures and their resistance to chemically aggressive environments increase. As a rule, dowel joints with steel gaskets are used in the nodes of glued elements of large cross-section.
It is much easier to make connections on nodal plates with a thickness of no more than 2 mm, which, without pre-drilling, can be pierced through with nails. These compounds include the Grame system. Here, metal plastics 1-1.75 mm thick are inserted into thin slots and punched through with nails.
Connections of wooden elements on thin plates of the Grame system: a - with trapezoidal plates; b - with triangular plates.
A plate located in a section inside a wooden element, when receiving nodal compressive forces, works on a longitudinal bend with a free length equal to the distance between the working bonds that fasten the plates to the wooden element. To prevent bulging of the plate, it is necessary to ensure its tight fit to the side edges of the cut and to establish working connections with a step at which the plate does not bulge.
Dowel joints with steel plates and spacers should be considered in the same way as ordinary dowel joints of wooden elements, determining the bearing capacity of the dowels from the condition of the dowel bending and the collapse of the wood in the dowel nest. In this case, in the calculation from the bending condition, the largest value of the bearing capacity of the dowel should be taken. Steel linings and gaskets should be checked for tension along the weakened section and for collapse under the dowel.
Nodal plates can also be made from other, in particular, layered materials. The most widespread connections of wooden elements on plates of bakelized plywood. They are mainly used for bonded and other connections, which are made directly at the construction site. Connections on plywood linings and gaskets are carried out on cylindrical dowels made of hardwood, steel, etc., on nails or screws. If plywood plates are located outside the wooden elements, then they are connected with single-cut dowels.
Multi-cut connections are also possible if the plates are installed in slots in wooden elements or between their individual branches. Glue based on synthetic resins is used to treat the edges of plywood sheets. Their thickness is chosen depending on the diameter of the dowel and from the conditions of the plywood for crushing in the nest. The latter are usually arranged so that the direction of the fibers of the outer layers of plywood coincides with the direction of the fibers of the connected element, in which there are large forces, or this angle is 45 °.
The development of pin connections with plates in the nodes led to the appearance of pin plates. One of the first to be used for nodal connections of structures with one or two branches was the pin plates of the Menig system. The plates of this system are made of 3 mm thick foam and a layer of synthetic resin reinforced with glass fiber 2 mm thick. Through double-edged dowels with a diameter of 1.6 mm and a length of 25 mm or more on each side of the plate are fixed in this plate. The thickness of the connected wooden elements can reach 80 mm.
Nail plates are installed between the connected wooden elements. During pressing, the foam layer is compressed and serves as a control for uniform pressing of the dowels into both connected elements.
In terms of their work, joints on dowel plates can be compared with the work of nail joints. The bearing capacity of joints on Menig-type plates is 0.75-1.5 N per 1 mm 2 of the contact surface.
Connections for large-section paving wooden elements on pin plates of high bearing capacity are metal plates with attached pins with a diameter of 3-4 mm. The pins can be through, pressed into the holes of the plate, or consist of two halves attached to both sides of the plate by spot welding.
The use of joints on dowel plates requires careful manufacturing, selection of material and pressing in special hydraulic presses with strict quality control.
Connections on metal toothed plates.
The most widespread in foreign construction practice was the MZP of the Gang-Neil system.
MZP are steel plates with a thickness of 1-2 mm, on one side of which, after stamping on special presses, teeth of various shapes and lengths are obtained. The MZP is placed in pairs on both sides of the elements to be joined in such a way that the rows of the MZP are located in the direction of the fibers of the attached wooden element, in which the greatest forces act.
Plank structures with connections on metal toothed plates should be used in buildings of the V degree of fire resistance without overhead handling equipment with temperature and humidity operating conditions A1, A2, B1 and B2. The manufacture of structures should be carried out at specialized enterprises or in woodworking shops equipped with equipment for assembling structures, pressing MZP and control testing of structures. Manual pressing of the MZP is unacceptable.
The bearing capacity of wooden structures at the MZP is determined by the conditions of crushing wood in nests and bending of the teeth of the plates, as well as by the conditions of the strength of the plates when working in tension, compression shear.
The material for the manufacture of structures is pine and spruce wood 100-200 mm wide, 40-60 mm thick. the quality of wood must meet the requirements of SNiP II-25-80 for materials of wooden structures.
MZP is recommended to be made of sheet carbon steel grades 08kp or 10kp according to GOST 1050-74 with a thickness of 1.2 and 2 mm. Anticorrosive protection of the MZP is performed by galvanizing in accordance with GOST 14623-69 or aluminum-based coatings in accordance with the recommendations for the anticorrosion protection of steel embedded parts and welded joints of prefabricated reinforced concrete. and concrete structures.
Wooden structures at joints with MZP count on the forces arising during the operation of buildings from permanent and temporary loads, as well as on the forces arising during the transportation and installation of structures. Through structures are calculated taking into account the continuity of the chords and assuming hinged fastening of the lattice elements to them.
The bearing capacity of the connection at the MZP N c, kN, according to the conditions of wood crushing and bending of the teeth in tension, shear and compression, when the elements perceive forces at an angle to the wood fibers, is determined by the formula:
where R is the calculated bearing capacity per 1 cm 2 of the working area of the joint, F p is the calculated surface area of the MZP on the butt element, determined minus the areas of the plate sections in the form of strips 10 mm wide adjacent to the interface lines of the elements and plate sections that are beyond outside the zone of rational location of the MZP, which is limited by lines parallel to the joint line, passing on both sides of it at a distance of half the length of the joint line.
Accounting for the eccentricity of applying forces to the MZP when calculating the support nodes of triangular trusses is carried out by reducing the calculated bearing capacity of the connection by multiplying by the coefficient h, determined depending on the slope of the upper chord. In addition, the plate itself is checked for tension and shear.
The bearing capacity of the MZP N p in tension is found by the formula:
where b is the size of the plate in the direction perpendicular to the direction of force, cm, Rp is the calculated tensile load bearing capacity of the plate, kN/m.
The bearing capacity of the MZP Q cf when shearing is determined by the formula:
Q av = 2l av R cp ,
where l cf is the cut length of the plate section without taking into account weakenings, cm, R cf is the calculated shear load-bearing capacity of the plate, kN/m.
Under the combined action of shear and tension forces on the plate, the following condition must be met:
(N p /2bR p) 2 + (Q av /2l av R cp) 2 £ 1.
When designing structures on the MZP, one should strive to unify the standard sizes of the MZP and lumber sections in one structure. On both sides of the nodal connection, MZP of the same standard size should be located. The connection area on each element (on one side of the connection plane) should be at least 50 cm 2 for structures with a span of up to 12 m, and at least 75 cm 2 for structures with a span of up to 18 m. The minimum distance from the plane of connection of the elements must be at least 60 mm. The MZP should be positioned in such a way that the distances from the side edges of the wooden elements to the extreme teeth are at least 10 mm.
Connections on stretched links.
Stretched bonds include nails, screws (screws and capercaillie), working for pulling out, staples, clamps, coupling bolts and cords. There are tension and non-tension connections, temporary (assembly) and permanent ones. All types of connections must be protected from corrosion.
Nails resist pulling out only by the forces of surface friction between them and the wood of the nest. Friction forces can decrease when cracks form in the wood, which reduce the compression force of the nail, therefore, for pull-out nails, it is imperative to comply with the same spacing standards that are accepted for nails working as bending pins (S 1 \u003d 15d, S 2, 3 = 4d).
With a static load application, the design bearing capacity for pulling out one nail hammered across the fibers in compliance with the placement standards is determined by the formula:
T vyd £ R vyd pd gv l protection,
where R vyd is the calculated pull-out resistance per unit of the contact surface of the nail with wood, d gv is the diameter of the nail, l protect is the estimated length of the pinched part of the nail that resists pulling out, m.
In wooden structures (for temporary structures) R vyd,. When determining Tvyd, the calculated diameter of the nail is taken to be no more than 5 mm, even if nails of greater thickness are used.
Estimated nail pinching length l zasch (excluding tip 1.5d) must be at least 10d and at least twice the thickness of the nailed board. In turn, the thickness of the nailed board must be at least 4d.
Screws (screws screwed with a screwdriver) and capercaillie (screws with a diameter of 12-20 cm, screwed with a wrench) are held in the wood not only by friction forces, but also by the emphasis of the screw thread in the screw grooves cut by it in the wood.
The arrangement of screws and capercaillie and the dimensions of the drilled sockets should ensure tight crimping of the capercaillie rod with wood without splitting it. S 1 \u003d 10d, S 2.3 \u003d 5d. The diameter of the part of the nest adjacent to the seam must exactly correspond to the diameter of the unthreaded part of the capercaillie rod. For a reliable stop of the screw thread of the capercaillie pulled out with screws, the diameter of the recessed part of the nest along the entire length of the threaded part of the capercaillie should be 2-4 mm less than its full diameter.
If during the design it is possible to allow a sparse arrangement of screws and capercaillie with a diameter of no more than 8-16 mm, then sockets are drilled with a diameter reduced by 2-3 mm for the entire length of the pinch.
If these requirements are met, the design bearing capacity for pulling out a screw or capercaillie is determined by the formula:
T ext £ R ext pd screw l protection,
where R vyd is the calculated resistance to pulling out of the uncut part of the screw or capercaillie, d screw is the outer diameter of the threaded part, m, l protect is the length of the threaded part of the screw or capercaillie, m.
All correction factors to Rvyd are entered in accordance with the corrections for the resistance to crushing across the fibers.
Capercaillie and screws are best used for attaching metal plates, clamps, washers, etc. to wooden beams and boards. At the same time, capercaillie and screws replace not only pins, but also coupling bolts. If, with the help of wood grouses or screws, wooden or plywood elements that work on separation are attached, it is not the resistance to pulling out the threaded part that becomes decisive, but the resistance to crushing the wood with the head of the wood grouse or screw. In this case, it is necessary to place a metal washer measuring 3.5d x 3.5d x 0.25d under the head.
Staples from round (or square) steel with a thickness of 10-18 mm, they are used as auxiliary stretched or fixing ties in structures made of round timber or beams, in bridge supports, scaffolding, log farms, etc. Staples are not used in plank wooden structures, as they split the boards. Staples are usually hammered with the ends into solid wood without drilling sockets. The load-bearing capacity of a single shackle, even with increased standards, is not certain.
Experimental studies have revealed the efficiency of driving without drilling staples from rolled cross profile d sk = 15 mm. With a sufficient length of the spike (6-7 d ck), the bearing capacity of such staples is approximately equal to the bearing capacity of a dowel made of round steel with a diameter of 15 mm.
Clamps , just like staples are stretched bonds. A distinctive feature of the clamps is their position in relation to the connected wooden elements.
Working bolts and ties, i.e. stretched metal elements are used as anchors, pendants, stretched elements of metal-wood structures, puffs of arched and vaulted structures, etc. All elements of strands and working bolts should be checked by calculation according to the standards for steel structures and should be taken with a diameter of at least 12 mm.
When determining the bearing capacity of tensile steel black bolts, weakened by threading, the reduced area F nt and the local stress concentration s p are taken into account; therefore, lower design resistances are accepted. The design resistance of steel in parallel-operating double or more strands and bolts is reduced by multiplying by a factor of 0.85, taking into account the uneven distribution of forces. In metal strands, local weakening of the working section should be avoided.
Working bolt connections and turnbuckles are used only in cases where installation or operational regulation of their length is required. They are located in the most accessible places of metal-wooden arches and trusses. Tension-free butt joint made of round steel, allowing it to be transported without dismantling.
Tension joints of round steel puffs, which are required only in rare cases, are carried out using clamping sleeves with multi-threaded threads. In the absence of factory-made couplings, welded couplings can be made from two (or better than 4) square nuts of the left and right threads, welded together with two steel strips.
Tie bolts, which are mainly of mounting importance and are not calculated on the perception of a certain operational force, are used in almost all types of joints, including dowel joints and cuts to ensure a snug fit of bonded boards, beams or logs. The cross section of the tie bolts is determined by installation considerations; it should be the greater, the thicker the elements of the connected node, i.e. the greater the expected resistance to straightening camber of warped or warped boards or beams. In the case of swelling of the wood of a package of boards tightly tightened with a bolt, the bolt core is subjected to large longitudinal tensile forces. To avoid at the same time the bolt breaking along the section weakened by cutting, the washers of the tie bolts are prescribed with a reduced wood crushing area. Connection-safe indentation of the washer into the wood. In the event of swelling, it must occur before the tensile stress of the bolt shaft reaches a dangerous value.
Collapsible joint with double crimp for stretched glued elements. Adhesive joints of stretched wooden elements were investigated by V.G. Mikhailov. The destruction of the joints occurred from splitting at low shear stresses along the fracture plane. The highest average shear stress at failure, equal to 2.4 MPa, was achieved at the junction with crimp wedges.
The joint with double crimping is covered with overlays 1 made of strip steel, to which corners 2 are welded. to stop the corners 6 in such a way that the shearing plane starting from the corner does not coincide with the glue line.
An analysis of tests of tensioned joints shows that the force that compresses the element at the beginning of the fracture plane during shearing, counteracting tensile stresses, simultaneously creates additional shear stresses and thereby increases their concentration in the hazardous area. When an additional crimping force across the fibers is created at the opposite end of the shearing plane (as is the case in the joint under consideration), shear stresses are equalized, their concentration and the possibility of tensile stresses occurring across the fibers decrease.
A joint with a double compression is a tension collapsible joint that creates an initial density and allows it to be maintained in the future under operating conditions (if some shrinkage of the connected elements occurs).
The joint for chipping on wood is calculated from the condition:
The average value of the design shear resistance is determined by the formula:
where b = 0.125; e = 0.125h.
Connections on glued steel rods working for pulling out or punching. The use of joints on glued rods made of rebar of a periodic profile with a diameter of 12-25 mm, working for pulling out and punching, is allowed under the operating conditions of structures at an ambient temperature of not more than 35 ° C.
Pre-cleaned and degreased rods are glued with epoxy-based compounds into drilled holes or into milled grooves. Hole diameters or groove sizes should be taken 5 mm larger than the diameters of the glued rods.
The design bearing capacity of such a rod for pulling out or punching along and across the fibers in the stretched and compressed joints of elements of wooden structures made of pine and spruce should be determined by the formula:
T \u003d R sc ×p × (d + 0.005) × l × k s,
where d is the diameter of the glued rod, m; l is the length of the embedded part of the rod, m, which should be taken according to the calculation, but not less than 10d and not more than 30d; k c - coefficient taking into account the uneven distribution of shear stresses depending on the length of the embedded part of the rod, which is determined by the formula: k c = 1.2 - 0.02×(l/d); Rsk is the calculated resistance of wood to chipping.
The distance between the axes of the glued rods, along the fibers, should be taken at least S 2 = 3d, and to the outer edges - at least S 3 = 2d.
Connections of DC elements on adhesives.
Requirements for adhesives for load-bearing structures.
Equal strength, solidity and durability of adhesive joints in wooden structures can only be achieved by using waterproof structural adhesives. The durability and reliability of the adhesive bond depends on the stability of adhesive bonds, the type of adhesive, its quality, gluing technology, operating conditions and surface treatment of the boards.
The glue line must provide a joint strength that is not inferior to the strength of wood, for chipping along the fibers and for stretching across the fibers. The strength of the adhesive joint, corresponding to the tensile strength of wood along the fibers, has not yet been obtained, therefore, in stretched joints, the area of glued surfaces has to be increased by about 10 times by oblique cutting of the butt by a mustache or a toothed spike.
The contact density of the adhesive with the surfaces to be bonded must be created even in the viscous-liquid phase of the structural adhesive, which fills all the recesses and roughness, due to the ability to wet the bonded surface. The smoother and cleaner the glued surfaces are and the tighter they adjoin each other, the more complete the solidity of the bonding, the more uniform and thinner the adhesive line. A wooden structure, solidly glued from dry thin boards, has a significant advantage over a beam cut from a single log, but to realize these advantages, strict adherence to all conditions of the technology of industrial production of glued wooden structures is necessary.
After the structural adhesive has cured, the formed adhesive joint requires not only equal strength and solidity, but also water resistance, heat resistance and biostability. During testing, the destruction of prototype adhesive joints should occur mainly along the glued wood, and not along the adhesive joint (with the destruction of internal, cohesive bonds) and not in the boundary layer between the adhesive joint and the material being glued (with the destruction of boundary, adhesive bonds).
Types of adhesives.
Adhesive joints have been used for a long time, mainly in joinery. At the beginning of the 20th century, load-bearing wooden structures on casein glue began to be used in Switzerland, Sweden and Germany. However, protein adhesives of animal origin, and even more so of vegetable origin, did not fully satisfy the requirements for joints of elements of load-bearing structures.
Of great importance is the development of the chemistry of polymeric materials and the production of synthetic adhesives. Synthetic polymer materials with planned properties provide the required strength and durability of adhesive joints. The search for the optimal range of structural adhesives and the corresponding modes of mass production of glued structures continues, but now there is a set of synthetic adhesives that allow you to connect wooden building parts not only with wood.
Unlike casein and other protein adhesives, synthetic structural adhesives form a strong, water-resistant adhesive seam as a result of a polymerization or polycondensation reaction. Currently, mainly resorcinol, phenol-resorcinol, alkylresorcinol, phenolic adhesives are used. According to SNiP II-22-80, the choice of the type of adhesive depends on the temperature and humidity conditions for the operation of glued structures.
The elasticity and viscosity of the adhesive joint is especially important when connecting wooden elements with metal, plywood, plastic and other structural elements that have temperature, shrinkage and elastic characteristics. However, the use of elastic rubber adhesives in stressed joints is usually unacceptable due to the insufficient strength of such joints and their excessive creep under prolonged loading.
The drier and thinner the boards to be glued, the less the risk of cracking in them. If the shrinkage warping of under-dried boards occurs even before the adhesive joint is cured, but after the pressure of the press is stopped, then the bonding will be irreversibly broken.
Types of joints on glue.
The stretched joint of glued elements in the factory is made on a toothed spike with a slope of the glued surfaces of about 1:10. This unified solution is not inferior in strength to the mustache joint solution (with the same slope), it is more economical in terms of wood consumption and more technologically advanced in production; therefore, it should completely replace all other types of joints during factory production.
The serrated spike works equally well in tension, bending, torsion and compression. According to tests, the strength of such a KB_3 joint, even at break, is not lower than the strength of a solid bar, weakened by a knot normal for category 1, ¼-1/6 of the width of the corresponding side of the element.
In practice, it is recommended to use the most technologically advanced option with cutting spikes perpendicular to the face. This option is applicable for any width of the elements to be glued, even slightly warped ones. When joining glued blocks of large cross sections, it is necessary to use cold (or warm) gluing.
For splicing plywood sheets in factory production, the same unified non-separable type of connection is a butt joint; its use in stressed structural elements requires compliance with the following conditions; the length of the mustache is taken equal to 10-12 plywood thicknesses, and the direction of the fibers of the outer veneers (shirts) must coincide with the direction of the acting forces. The weakening of ordinary plywood by the joint at the us is taken into account by the coefficient K osl \u003d 0.6, and the weakening of bakelized plywood by the coefficient 0.8.
Adhesive and glue-mechanical connections of elements in structures with the use of plastics and the principles of their calculation.
Adhesive joints are the most effective, versatile and common plastic joints. Give the chance to stick together any materials and plastics. Disadvantage of adhesive bonding: low transverse tensile strength - tear-off and limited heat resistance. Thermosetting and thermoplastic adhesives are used.
Connection types see fig. The length of the glue joint on each side of the joint (the length of the overlap) is determined by calculating it per cut, but not less than 8 sheet thicknesses for asbestos cement, 50 sheet thicknesses for metals, 20 sheet thicknesses for fiberglass. Adhesive joints most often work in shear, but in some cases the joint may experience forces that cause tension in it, which is called separation. Depending on the nature of the distribution of tensile stresses along the length of the seam, uniform and uneven separation is distinguished. More often, the strength of the adhesive layer is higher than the strength of the material being glued, in this case, the calculated resistance is determined by the material to be joined. For adhesive joints, the coefficients of the working conditions are taken into account: temperature factor; humidity conditions; atmospheric conditions. 
Adhesive metal joints are combined, consisting of point metal joints and an adhesive layer located along the entire seam. There are glue-welded, glue-screw, glue-riveting. They have higher strength with uneven separation. When sheared, they are stronger than metal joints. The shear strength of adhesive metal joints is defined as the strength of a rivet, screw, or spot weld multiplied by a factor of 1.25-2, which takes into account the performance of the adhesive. The strength of a rivet, screw is determined from the crushing or shear condition, and the strength of the weld point from the shear condition.
Welded joints of plastic elements and principles of their calculation.
Welded plastic joints are used to connect elements of the same thermoplastic material. Welding is carried out due to the simultaneous action of high temperature and pressure. Advantages: high density of the seam, speed of their implementation, simplicity of technological operations. There are two welding methods: welding in a stream of hot air (similar to gas welding of metals) and a contact method (used when welding plexiglass, vinyl plastic, polyethylene). 1) The material and the filler rod are softened in a stream of hot air heated to 250º. A heat gun is used as a source of warm air. 2) For the device of the weld according to one of the variants of the contact method, the points of contact of the two parts to be joined are cut off with a slope of 1: 3 ... 1: 5, aligned along the contact area and fixed in this position. The seam is then compressed and heated. The strength of the weld is lower than the strength of the material. For vinyl plastic, the decrease in strength is 15-35% in compression, tension and bending, and when tested for specific impact strength, the strength decreases by 90%.
Types of Composite Bars and Accounting for the Compliance of Bonds in Their Calculation for Central Compression.
Compliance- the ability of connections during the deformation of structures to enable the connected bars or boards to move one relative to the other.
Types of composite rods: package rods; rods with short spacers; rods, some of the branches of which are not supported at the ends.
Rod packages. All branches of such rods are supported at the ends and perceive the compressive force, and the distances between the bonds along the length of the rod are small and do not exceed seven branch thicknesses. The calculation relative to the x-x axis, perpendicular to the seams between the branches, is carried out as for a solid section, since in this case the flexibility of the composite rod is equal to the flexibility of a separate branch. The calculation relative to the y-y axis, parallel to the seams, is performed taking into account the compliance of the bonds. With a small distance between the bonds along the length of the rod, equal to the free length of the branch, the area of supported branches;
The ductility of the bonds impairs the operation of a composite element compared to the same element of a solid section. For a composite element on pliable bonds, the bearing capacity decreases, deformability increases, the nature of the distribution of shear forces along its length changes, therefore, when calculating and designing composite elements, it is necessary to take into account the compliance of bonds.
Consider three wooden beams whose loads, spans and cross sections are the same. Let the load of these beams be uniformly distributed. The first solid section beam, i.e. consists of one beam. Let's call this beam C. The moment of inertia of the cross section of the beam I c \u003d bh 3 / 12; moment of resistance W c \u003d bh 2 /6; deflection
f c \u003d 5q n l 4 / 384EI c.
The second beam P of the composite section consists of two beams connected by flexible connections, such as bolts. The moments of inertia and its resistance, respectively, will be I p and W p; deflection f p.
The third beam O of the composite section consists of the same beams as the second beam, but there are no connections here and therefore both beams will work independently. The moment of inertia of the third beam I o = bh 3 /48, which is 4 times less than beams of solid section. The moment of resistance W o \u003d bh 2 /12, which is 2 times less than beams of a solid section. Deflection f o \u003d 5q n l 4 / 384EI o, which is 4 times greater than the deflection of a solid section beam.
Consider what will happen on the left support of the beam when it is deformed under load. The left support of a solid section beam will rotate by an angle j, and for a beam of a composite section without connections, in addition to turning on the left support, a shift d about the upper beam relative to the lower one will occur.
In a composite beam on pliable ties, the beams will be prevented from moving by bolts, so it is less here than in a beam without ties. Consequently, a composite beam with pliable braces occupies an intermediate position between a solid section beam and a composite beam without braces. Therefore, you can write: I c\u003e I p\u003e I o; W c > W p > W o; f c
It follows from these inequalities that the geometric characteristics of a composite beam on pliable ties I c, W p can be expressed through the geometric characteristics of a solid section beam, multiplied by coefficients less than one, which take into account the compliance of the bonds: I p \u003d k w I c and W p \u003d k w W c, where k w and k w vary from 1 to I o /I c and from 1 to W o /W c, respectively (with two bars I o /I c = 0.25, and W o / W c = 0.5.
The deflection of the beam increases in accordance with the decrease in the moment of inertia f p \u003d f c / k well.
The calculation of a composite beam on pliable ties is thus reduced to the calculation of a solid section beam with the introduction of coefficients that take into account the ductility of the ties. Normal stresses are determined by the formula: s and \u003d M / W c k w £ R and, where W c is the moment of resistance of the composite beam as a whole; k w is a coefficient less than one, taking into account the compliance of bonds.
The deflection of a composite beam on pliable bonds is determined by the formula: f p \u003d 5q n l 4 /384EI c k w £ f pr, where I c is the moment of resistance of the beam as a whole; k w - coefficient less than one, taking into account the compliance of the bonds.
The value of the coefficients k w and k w are given in SNiP II-25-80 “Wooden structures. Design standards”.
The number of connections is determined by the calculation of the shear force. The shear force T over the entire width of the beam, equal to tb, is calculated by the formula: T \u003d QS / I.
The distribution of shear forces along the length is similar to the distribution of shear stresses in the form of a straight line passing at an angle along the horizontal. The total shear force of the beam in the section from the support to the point where T \u003d 0 will be geometrically equal to the area of the triangle. In our case, with a uniformly distributed load T = 0, if x = l/2, and then the total shear force H = M max S/I.
In a composite beam on yielding bonds, the value of the total shear force remains constant. However, due to the flexibility of the bonds, the nature of the distribution of shear forces along the length of the beam will change. As a result of the shift of the bars, the triangular diagram will turn into a curvilinear one, close to a cosine wave. If the links are placed uniformly along the length of the beam, then each link can perceive a shear force equal to its bearing capacity T c, and all of them must perceive the full shear force. Thus, n c T c = M max S/I.
The work of such a number of connections will correspond to the ADEC rectangle, i.e. communications located near the supports will be overloaded. Therefore, when calculating the number of links, two conditions must be met:
the number of evenly placed bonds on the section of the beam from the support to the section with a maximum moment must take the full shear force
n c = M max S/IT c ;
· Connections placed near the supports should not be overloaded.
The connections near the supports are overloaded by 1.5 times, therefore, to comply with the second condition, it is necessary to increase their number by 1.5 times. Thus, the required number of bonds in the section of the beam from the supports to the section with the maximum moment will be n c = 1.5M max S/I br T c .
The calculation method for compressively-bent elements of a composite section on pliable bonds remains the same as for elements of a solid section, but the ductility of the bonds is additionally taken into account in the formulas.
When calculating in the bending plane, the composite element experiences complex resistance, and the ductility of the bonds is taken into account twice:
· the introduction of the coefficient k w , the same as in the calculation of composite elements for transverse bending;
· calculation of the coefficient x, taking into account the reduced flexibility of the element.
Normal stress is determined by the formula:
s c \u003d N / F nt + M d / W nt k w £ R c , where M d \u003d M q / x and x \u003d 1 - l p 2 N / 3000F br R c ; l p \u003d ml c;
where k c is the coefficient of ductility of the joints, the shift of bonds obtained from experimental data; b is the width of the integral part of the cross section, cm; h is the total height of the cross section, cm; l calc - the estimated length of the element, m; n w - number of shear seams; n c - the number of cuts of bonds in 1 m of one seam, with several seams with a different number of cuts of bonds, the average number of bonds is taken.
Deflection f p \u003d 5q n l 4 / 384EIk w x £ f pr.
When determining the number of bonds that must be placed in the section from the support to the section with the maximum moment, the increase in the transverse force with a compressed-bent element n c \u003d 1.5M max S / IT c x .. is taken into account.
Compressively-bent elements are calculated from the bending plane approximately without taking into account the bending moment, i.e. as centrally compressed composite rods.
"Artificial satellites of the Earth" - Does the Earth have a natural satellite? Evening. Connect two circles with a long bar. Monitor the state of forests, fields, fires. The results obtained are recorded in a notebook. Observer satellites. Mutual attraction of the Sun and the Earth. People have learned how to launch satellites into orbit. What is the topic of the lesson? Research satellites.
"Organic wool" - Sizes: Height 44, premature, small Height 50, 0-3 months. Height 86, 1-2 years Cap and hat-helmet. Keep the baby in comfortable warmth and does not hinder movement. Envelope for a car seat. Height 44, premature, small Height 50, 0-3 months. The outer seam does not irritate children's skin. The energy of wool is similar to the energy of mom.
"Spike connections" - Lugs and sockets are obtained using chisels and chisels. Pins are used to strengthen joints. Spikes and lugs are marked on both sides of the workpiece. Of the adhesive joints, stud joints are the most common. The diameter of the drill should be equal to the diameter of the dowel. Parts and dowels are made by machine operators there, and assembled by assemblers.
"Organic substances" - The subject of organic chemistry. Compare this concept with the concept of "oxidation state". The structure of the C3 H8 propane molecule is reflected by the formulas: Give specific examples. Valence. For example, the chemical structure of methane: 3. Theory of chemical structure. 4. Questions and tasks. Structural formula. Abbreviated structural formula.
"Development of organic chemistry" - Azimov A.N. Brief history of chemistry. Lectures. To trace the evolution of chemical ideas and ideas in the period from prehistory to the present. Trends in the development of organic chemistry. Presentation. Get acquainted with the achievements, current state and prospects for the development of chemistry. Handicraft organic chemistry: brewing, winemaking, making drugs, dyes.
"Artificial selection Darwin" - Breeding by breeders of 150 breeds of pigeons, many breeds of dogs, varieties of cabbage ... Charles Darwin's teaching on artificial selection. selection methods. Laboratory work "Comparison of animal breeds". Ch. Darwin's study of the practice of agriculture in England. Artificial selection is the process of creating new breeds of animals and varieties of cultivated plants through the systematic selection and reproduction of individuals with certain traits and properties that are valuable to humans.
If you ask scientists, which of the discoveries of the XX century. most important, then hardly anyone will forget to name the artificial synthesis of chemical elements. In a short period of time - less than 40 years - the list of known chemical elements has increased by 18 names. And all 18 were synthesized, prepared artificially.
The word "synthesis" usually means the process of obtaining from a simple complex. For example, the interaction of sulfur with oxygen is the chemical synthesis of sulfur dioxide SO 2 from the elements.
Synthesis of elements can be understood in this way: artificial production of an element with a lower nuclear charge, a lower serial number of an element with a higher serial number from an element with a lower nuclear charge. And the process of obtaining is called a nuclear reaction. Its equation is written in the same way as the equation of an ordinary chemical reaction. The reactants are on the left and the products are on the right. The reactants in a nuclear reaction are the target and the bombarding particle.
The target can be any element of the periodic system (in free form or in the form of a chemical compound).
The role of bombarding particles is played by α-particles, neutrons, protons, deuterons (nuclei of the heavy isotope of hydrogen), as well as the so-called multiply charged heavy ions of various elements - boron, carbon, nitrogen, oxygen, neon, argon and other elements of the periodic system.
For a nuclear reaction to occur, the bombarding particle must collide with the nucleus of the target atom. If the particle has a sufficiently high energy, then it can penetrate so deeply into the nucleus that it merges with it. Since all the particles listed above, except for the neutron, carry positive charges, then, merging with the nucleus, they increase its charge. And changing the value of Z means the transformation of elements: the synthesis of an element with a new value of the nuclear charge.
In order to find a way to accelerate the bombarding particles, to give them high energy sufficient for their fusion with nuclei, a special particle accelerator, the cyclotron, was invented and constructed. Then they built a special factory of new elements - a nuclear reactor. Its direct purpose is to generate nuclear energy. But since there are always intense neutron fluxes in it, they are easy to use for the purposes of artificial synthesis. The neutron has no charge, and therefore it is not necessary (and impossible) to accelerate. On the contrary, slow neutrons turn out to be more useful than fast ones.
Chemists had to rack their brains and show genuine miracles of ingenuity in order to develop ways to separate negligible amounts of new elements from the target substance. Learn to study the properties of new elements when only a few of their atoms were available...
Through the work of hundreds and thousands of scientists, eighteen new cells were filled in the periodic table.
Four are within its old boundaries: between hydrogen and uranium.
Fourteen - for uranium.
Here's how it all happened...
Technetium, promethium, astatine, francium... Four places in the periodic table remained empty for a long time. These were cells No. 43, 61, 85 and 87. Of the four elements that were supposed to take these places, three were predicted by Mendeleev: ekamanganese - 43, ekaiod - 85 and ekacesium - 87. The fourth - No. 61 - should have belonged to rare earth elements .
These four elements were elusive. The efforts of scientists aimed at searching for them in nature remained unsuccessful. With the help of the periodic law, all other places in the periodic table have long been filled - from hydrogen to uranium.
More than once in scientific journals there were reports of the discovery of these four elements. Ecamarganese was "discovered" in Japan, where it was given the name "nipponium", in Germany it was called "masurium". Element No. 61 was "discovered" in different countries at least three times, it received the names "illinium", "Florence", "onium cycle". Ekaiod was also found in nature more than once. He was given the names "Alabamy", "Helvetius". Ekacesium, in turn, received the names "Virginia", "Moldavia". Some of these names ended up in various reference books and even found their way into school textbooks. But all these discoveries were not confirmed: each time an exact check showed that a mistake had been made, and random insignificant impurities were mistaken for a new element.
A long and difficult search finally led to the discovery in nature of one of the elusive elements. It turned out that ecacesium, which should occupy the 87th place in the periodic table, occurs in the decay chain of the natural radioactive isotope uranium-235. It is a short lived radioactive element.
Element number 87 deserves to be told in more detail.
Now in any encyclopedia, in any textbook on chemistry we read: francium (serial number 87) was discovered in 1939 by the French scientist Marguerite Perey. By the way, this is the third case when the honor of discovering a new element belongs to a woman (previously Marie Curie discovered polonium and radium, Ida Noddack discovered rhenium).
How did Perey manage to capture the elusive element? Let's go back many years. In 1914, three Austrian radiochemists - S. Meyer, W. Hess and F. Panet - began to study the radioactive decay of the actinium isotope with a mass number of 227. It was known that it belongs to the actinouranium family and emits β-particles; hence its decay product is thorium. However, scientists had a vague suspicion that actinium-227, in rare cases, also emits α-particles. In other words, one of the examples of a radioactive fork is observed here. It is easy to imagine that in the course of such a transformation, an isotope of element No. 87 should be formed. Meyer and his colleagues actually observed α-particles. Further studies were required, but they were interrupted by the First World War.
Marguerite Perey followed the same path. But she had at her disposal more sensitive instruments, new, improved methods of analysis. That is why she was successful.
Francium is one of the artificially synthesized elements. But still, the element was first discovered in nature. It is an isotope of francium-223. Its half-life is only 22 minutes. It becomes clear why there is so little France on Earth. Firstly, because of its fragility, it does not have time to concentrate in any noticeable quantities, and secondly, the process of its formation itself is characterized by a low probability: only 1.2% of actinium-227 nuclei decays with the emission of α-particles.
In this regard, francium is more profitable to prepare artificially. Already received 20 isotopes of francium, and the longest-lived of them - francium-223. Working with absolutely negligible amounts of francium salts, chemists were able to prove that in its properties it is extremely similar: to cesium.
Elements #43, 61 and 85 remained elusive. In nature, they could not be found in any way, although scientists already possessed a powerful method that unmistakably points the way for the search for new elements - the periodic law. Thanks to this law, all the chemical properties of an unknown element were known to scientists in advance. So why were the searches for these three elements in nature unsuccessful?
Studying the properties of atomic nuclei, physicists came to the conclusion that elements with atomic numbers 43, 61, 85 and 87 cannot have stable isotopes. They can only be radioactive, with short half-lives, and should disappear quickly. Therefore, all these elements were created by man artificially. The paths for creating new elements were indicated by the periodic law. Let's try with its help to outline the route for the synthesis of ecamarganese. This element number 43 was the first artificially created.
The chemical properties of an element are determined by its electron shell, and it depends on the charge of the atomic nucleus. There should be 43 positive charges in the nucleus of element 43, and 43 electrons should revolve around the nucleus. How can you create an element with 43 charges in the atomic nucleus? How can one prove that such an element has been created?
Let us consider carefully which elements in the periodic system are located near the empty space intended for element No. 43. It is located almost in the middle of the fifth period. In the corresponding places in the fourth period is manganese, and in the sixth - rhenium. Therefore, the chemical properties of the 43rd element should be similar to those of manganese and rhenium. No wonder D. I. Mendeleev, who predicted this element, called it ecamarganese. To the left of cell 43 is molybdenum, which occupies cell 42, to the right, in cell 44, ruthenium.
Therefore, in order to create element number 43, it is necessary to increase the number of charges in the nucleus of an atom, which has 42 charges, by one more elementary charge. Therefore, for the synthesis of a new element No. 43, molybdenum must be taken as a feedstock. It has 42 charges in the core. The lightest element, hydrogen, has one positive charge. So, it can be expected that element No. 43 can be obtained as a result of a nuclear reaction between molybdenum and hydrogen.
The properties of element No. 43 must be similar to those of manganese and rhenium, and in order to detect and prove the formation of this element, one must use chemical reactions similar to those by which chemists determine the presence of small amounts of manganese and rhenium. This is how the periodic table makes it possible to chart the way for the creation of an artificial element.
In exactly the same way that we have just outlined, the first artificial chemical element was created in 1937. He received a significant name - technetium - the first element made by technical, artificial means. This is how technetium was synthesized. The plate of molybdenum was subjected to intense bombardment by nuclei of the heavy isotope of hydrogen - deuterium, which were dispersed in the cyclotron to great speed.
The nuclei of heavy hydrogen, which received very high energy, penetrated into the nuclei of molybdenum. After irradiation in the cyclotron, the molybdenum plate was dissolved in acid. An insignificant amount of a new radioactive substance was isolated from the solution using the same reactions that are necessary for the analytical determination of manganese (analogue of element No. 43). This was the new element, technetium. Soon its chemical properties were studied in detail. They correspond exactly to the position of the element in the periodic table.
Now technetium has become quite affordable: it is formed in fairly large quantities in nuclear reactors. Technetium has been well studied and is already being used in practice. Technetium is used to study the process of corrosion of metals.
The method by which the 61st element was created is very similar to the method by which technetium is obtained. Element #61 must be a rare earth element: the 61st cell is between neodymium (#60) and samarium (#62). The new element was first obtained in 1938 in a cyclotron by bombarding neodymium with deuterium nuclei. Element 61 was chemically isolated only in 1945 from fragmentation elements formed in a nuclear reactor as a result of uranium fission.
The element received the symbolic name promethium. This name was given to him for a reason. The ancient Greek myth tells that the titan Prometheus stole fire from the sky and gave it to people. For this he was punished by the gods: he was chained to a rock, and a huge eagle tormented him every day. The name "promethium" not only symbolizes the dramatic path of science stealing the energy of nuclear fission from nature and mastering this energy, but also warns people against a terrible military danger.
Promethium is now obtained in considerable quantities: it is used in atomic batteries - sources of direct current, capable of operating without interruption for several years.
The heaviest halogen ekaiod element No. 85 was also synthesized in a similar way. It was first obtained by bombarding bismuth (No. 83) with helium nuclei (No. 2), accelerated in a cyclotron to high energies.
The nuclei of helium, the second element in the periodic table, have two charges. Therefore, for the synthesis of the 85th element, bismuth, the 83rd element, was taken. The new element is named astatine (unstable). It is radioactive and disappears quickly. Its chemical properties also turned out to correspond exactly to the periodic law. It looks like iodine.
transuranium elements.
Chemists have put a lot of work into searching for elements heavier than uranium in nature. More than once triumphant announcements appeared in scientific journals about the "reliable" discovery of a new "heavy" element with an atomic mass greater than that of uranium. For example, element No. 93 was "discovered" in nature many times, it received the names "bohemia", "sequania". But these "discoveries" turned out to be the result of errors. They characterize the difficulty of precise analytical determination of insignificant traces of a new unknown element with unexplored properties.
The result of these searches was negative, because there are practically no elements on Earth corresponding to those cells of the periodic table that should be located beyond the 92nd cell.
The first attempts to artificially obtain new elements heavier than uranium are associated with one of the most remarkable mistakes in the history of the development of science. It was noticed that under the influence of the neutron flux, many elements become radioactive and begin to emit β-rays. The nucleus of an atom, having lost a negative charge, shifts one cell to the right in the periodic system, and its serial number becomes one more - a transformation of elements occurs. Thus, under the influence of neutrons, heavier elements are usually formed.
They tried to act on uranium with neutrons. Scientists hoped that, like other elements, uranium would also have β-activity and, as a result of β-decay, a new element with a number greater than one would appear. It is he who will occupy the 93rd cell in the Mendeleev system. It was suggested that this element should be similar: to rhenium, so it was previously called ecarium.
The first experiments seemed to immediately confirm this assumption. Even more, it was found that in this case, not one new element arises, but several. Five new elements heavier than uranium have been reported. In addition to ecarium, ekaosmium, ekairidium, ekaplatinum and ekazoloto were "discovered". And all the discoveries turned out to be a mistake. But that was a remarkable mistake. It led science to the greatest achievement of physics in the history of mankind - to the discovery of the fission of uranium and the mastery of the energy of the atomic nucleus.
No transuranic elements have actually been found. With strange new elements, attempts were made in vain to find the supposed properties that the elements from ecarium and ecagold should have. And suddenly, among these elements, radioactive barium and lanthanum were unexpectedly discovered. Not transuranium, but the most common, but radioactive isotopes of elements, the places of which are in the middle of the periodic system of Mendeleev.
A little time passed, and this unexpected and very strange result was correctly understood.
Why, from the atomic nuclei of uranium, which is at the end of the periodic system of elements, under the action of neutrons, nuclei of elements are formed, the places of which are in its middle? For example, under the action of neutrons on uranium, elements appear corresponding to the following cells of the periodic system:
Many elements have been found in the unimaginably complex mixture of radioactive isotopes produced in neutron-irradiated uranium. Although they turned out to be old, long-familiar elements to chemists, at the same time they were new substances, first created by man.
In nature, there are no radioactive isotopes of bromine, krypton, strontium, and many other of the thirty-four elements - from zinc to gadolinium, that arise when uranium is irradiated.
It often happens in science: the most mysterious and most complex turns out to be simple and clear when it is unraveled and understood. When a neutron hits a uranium nucleus, it splits, splits into two fragments - into two atomic nuclei of smaller mass. These fragments can be of various sizes, which is why so many different radioactive isotopes of ordinary chemical elements are formed.
One atomic nucleus of uranium (92) decays into atomic nuclei of bromine (35) and lanthanum (57), fragments during the splitting of another may turn out to be atomic nuclei of krypton (36) and barium (56). The sum of the atomic numbers of the resulting fragmentation elements will be equal to 92.
This was the beginning of a chain of great discoveries. It was soon discovered that under the impact of a neutron, not only fragments arise from the nucleus of an atom of uranium-235 - nuclei with a lower mass, but also two or three neutrons fly out. Each of them, in turn, is capable of again causing the fission of the uranium nucleus. And with each such division, a lot of energy is released. This was the beginning of man's mastery of intra-atomic energy.
Among the huge variety of products arising from the irradiation of uranium nuclei with neutrons, the first real transuranium element No. 93, which remained unnoticed for a long time, was subsequently discovered. It arose under the action of neutrons on uranium-238. In terms of chemical properties, it turned out to be very similar to uranium and was not at all similar: to rhenium, as was expected during the first attempts to synthesize elements heavier than uranium. Therefore, they could not immediately detect it.
The first man-made element outside the "natural system of chemical elements" was named neptunium, after the planet Neptune. Its creation has expanded for us the boundaries defined by nature itself. Likewise, the predicted discovery of the planet Neptune has expanded the boundaries of our knowledge of the solar system.
Soon the 94th element was also synthesized. It was named after the last planet. solar system.
They called it plutonium. In Mendeleev's periodic system, it follows neptunium in order, similarly to "the last planet of the Solar * system, Pluto, whose orbit lies beyond the orbit of Neptune. Element No. 94 arises from neptunium during its β-decay.
Plutonium is the only transuranium element that is now produced in nuclear reactors in very large quantities. Like uranium-235, it is capable of fission under the influence of neutrons and is used as fuel in nuclear reactors.
Elements 95 and 96 are called americium and curium. They are also now produced in nuclear reactors. Both elements have very high radioactivity - they emit α-rays. The radioactivity of these elements is so great that concentrated solutions of their salts heat up, boil and glow very strongly in the dark.
All transuranium elements - from neptunium to americium and curium - were obtained in fairly large quantities. In its pure form, these are silver-colored metals, all of them are radioactive and, in terms of chemical properties, are somewhat similar to each other, and in some ways they differ noticeably.
The 97th element, berkelium, was also isolated in its pure form. To do this, it was necessary to place a pure preparation of plutonium inside a nuclear reactor, where it was exposed to a powerful neutron flux for six whole years. During this time, several micrograms of element No. 97 accumulated in it. Plutonium was removed from a nuclear reactor, dissolved in acid, and the longest-lived berkelium-249 was isolated from the mixture. It is highly radioactive - it decays by half in a year. So far, only a few micrograms of Berkelium have been obtained. But this amount was enough for scientists to accurately study its chemical properties.
Element number 98 is very interesting - californium, the sixth after uranium. Californium was first created by bombarding a curium target with alpha particles.
The history of the synthesis of the next two transuranium elements: 99th and 100th is fascinating. For the first time they were found in the clouds and in the "mud". To study what is formed in thermonuclear explosions, the aircraft flew through the explosive cloud, and sediment samples were collected on paper filters. Traces of two new elements were found in this sediment. To obtain more accurate data, a large amount of "dirt" was collected at the site of the explosion - soil and rock changed by the explosion. This "dirt" was processed in the laboratory, and two new elements were isolated from it. They were named einsteinium and fermium, in honor of the scientists A. Einstein and E. Fermi, to whom humanity is primarily obliged by the discovery of ways to master atomic energy. Einstein owns the law of equivalence of mass and energy, and Fermi built the first atomic reactor. Now einsteinium and fermium are also obtained in laboratories.
Elements of the second hundred.
Not so long ago, hardly anyone could believe that the symbol of the hundredth element would be included in the periodic table.
The artificial synthesis of elements did its job: for a short time, fermium closed the list of known chemical elements. The thoughts of scientists were now directed into the distance, to the elements of the second hundred.
But on the way there was a barrier, which was not easy to overcome.
So far, physicists have been synthesizing new transuranium elements mainly in two ways. Or they fired at targets from transuranium elements, already synthesized, with α-particles and deuterons. Or they bombarded uranium or plutonium with powerful neutron fluxes. As a result, isotopes of these elements very rich in neutrons were formed, which, after several successive β-decays, turned into isotopes of new transuraniums.
However, in the mid-1950s, both of these possibilities were exhausted. In nuclear reactions, it was possible to obtain imponderable amounts of einsteinium and fermium, and therefore it was impossible to make targets from them. The neutron method of synthesis also did not allow one to advance beyond fermium, since the isotopes of this element underwent spontaneous fission with a much higher probability than β decay. It is clear that under such conditions it made no sense to talk about the synthesis of a new element.
Therefore, physicists took the next step only when they managed to accumulate the minimum amount of element No. 99 required for the target. This happened in 1955.
One of the most remarkable achievements that science can rightfully be proud of is the creation of the 101st element.
This element was named after the great creator of the periodic table of chemical elements, Dmitri Ivanovich Mendeleev.
Mendelevium was obtained in the following way. An invisible coating of approximately one billion einsteinium atoms was applied to a sheet of the thinnest gold foil. Alpha particles with very high energy, breaking through the gold foil from the reverse side, colliding with einsteinium atoms could enter into a nuclear reaction. As a result, atoms of the 101st element were formed. With such a collision, the mendelevium atoms flew out from the surface of the gold foil and collected on another, located next to it, the thinnest gold leaf. In this ingenious way, it was possible to isolate the pure atoms of element 101 from a complex mixture of einsteinium and its decay products. Invisible plaque was washed off with acid and subjected to radiochemical research.
Truly it was a miracle. The source material for the creation of the 101st element in each individual experiment was approximately one billion einsteinium atoms. This is very little less than one billionth of a milligram, and it was impossible to obtain einsteinium in larger quantities. It was calculated in advance that out of a billion einsteinium atoms, under many hours of bombardment with α-particles, only one single atom of einsteinium can react and, consequently, only one atom of a new element can be formed. It was necessary not only to be able to detect it, but also to do it in such a way as to find out from just one atom the chemical nature of the element.
And it was done. The success of the experiment exceeded calculations and expectations. It was possible to notice in one experiment not one, but even two atoms of a new element. In total, seventeen mendelevium atoms were obtained in the first series of experiments. This turned out to be enough to establish both the fact of the formation of a new element and its place in the periodic system and to determine its basic chemical and radioactive properties. It turned out that this is an α-active element with a half-life of about half an hour.
Mendelevium - the first element of the second hundred - turned out to be a kind of milestone on the way to the synthesis of transuranium elements. Until now, it remains the last of those that were synthesized by the old methods - irradiation with α-particles. Now more powerful projectiles have entered the scene - accelerated multiply charged ions of various elements. Determination of the chemical nature of mendelevium by a counted number of its atoms laid the foundation for a completely new scientific discipline - the physicochemistry of single atoms.
The symbol of element No. 102 No - in the periodic system is taken in brackets. And in these brackets lies a long and complicated history of this element.
The synthesis of nobelium was reported in 1957 by an international group of physicists working at the Nobel Institute (Stockholm). For the first time, heavy accelerated ions were used to synthesize a new element. They were 13 C ions, the flow of which was directed to the curium target. The researchers came to the conclusion that they managed to synthesize an isotope of the 102nd element. He was given the name in honor of the founder of the Nobel Institute, the inventor of dynamite, Alfred Nobel.
A year has passed, and the experiments of the Stockholm physicists were reproduced almost simultaneously in the Soviet Union and the USA. And an amazing thing turned out: the results of Soviet and American scientists had nothing in common either with the work of the Nobel Institute or with each other. No one and nowhere else has been able to repeat the experiments carried out in Sweden. This situation gave rise to a rather sad joke: "There is only one No left from Nobel" (No - translated from English means "no"). The symbol, hastily placed on the periodic table, did not reflect the actual discovery of the element.
A reliable synthesis of element No. 102 was made by a group of physicists from the Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research. In 1962-1967. Soviet scientists synthesized several isotopes of element No. 102 and studied its properties. Confirmation of these data was obtained in the United States. However, the symbol No, having no right to do so, is still in the 102nd cell of the table.
Lawrencium, element No. 103 with the symbol Lw, named after the inventor of the cyclotron E. Lawrence, was synthesized in 1961 in the USA. But here the merit of the Soviet physicists is no less. They obtained several new isotopes of lawrencium and studied the properties of this element for the first time. Lawrencium also came into being through the use of heavy ions. The Californian target was irradiated with boron ions (or the americium target with oxygen ions).
Element No. 104 was first obtained by Soviet physicists in 1964. Bombardment of plutonium with neon ions led to its synthesis. The 104th element was named kurchatovium (symbol Ki) in honor of the outstanding Soviet physicist Igor Vasilyevich Kurchatov.
The 105th and 106th elements were also synthesized for the first time by Soviet scientists - in 1970 and in 1974. The first of these, the product of the bombardment of americium with neon ions, was named nilsborium (Ns) in honor of Niels Bohr. The synthesis of the other was carried out as follows: a lead target was bombarded with chromium ions. Syntheses of elements 105 and 106 were also carried out in the USA.
You will learn about this in the next chapter, and we will conclude the present one with a short story about how
how to study the properties of the elements of the second hundred.
A fantastically difficult task confronts experimenters.
Here are its initial conditions: a few quantities (tens, at best hundreds) of atoms of a new element are given, and atoms are very short-lived (half-lives are measured in seconds, or even fractions of a second). It is required to prove that these atoms are atoms of a really new element (i.e., to determine the value of Z, as well as the value of the mass number A, in order to know which isotope of the new transuranium is in question), and to study its most important chemical properties.
A few atoms, a tiny lifespan...
Scientists come to the aid of speed and the highest ingenuity. But a modern researcher - a specialist in the synthesis of new elements - must not only be able to "shoe a flea". He must also be fluent in theory.
Let us follow the basic steps by which a new element is identified.
The most important business card is primarily radioactive properties - this can be the emission of α-particles or spontaneous fission. Each α-active nucleus is characterized by specific energies of α-particles. This circumstance makes it possible either to identify known nuclei or to conclude that new ones have been discovered. For example, by studying the features of α-particles, scientists were able to obtain reliable evidence of the synthesis of the 102nd and 103rd elements.
The energetic fragmentation nuclei formed as a result of fission are much easier to detect than α-particles, due to the much higher energy of the fragments. For their registration, plates made of glass of a special grade are used. The fragments leave slightly noticeable traces on the surface of the plates. The plates are then chemically treated (etched) and carefully examined under a microscope. Glass dissolves in hydrofluoric acid.
If a glass plate, fired with fragments, is placed in a solution of hydrofluoric acid, then in places where the fragments have fallen, the glass will dissolve faster and holes will form there. Their dimensions are hundreds of times larger than the original trace left by the fragment. The wells can be observed under a microscope at low magnification. Other radioactive emissions cause less damage to glass surfaces and are not visible after etching.
Here is what the authors of the synthesis of kurchatovium tell about how the process of identifying a new element took place: “An experiment is underway. For forty hours, neon nuclei are continuously bombarding a plutonium target. For forty hours, the tape carries synthetic nuclei to glass plates. Finally, the cyclotron is turned off. "We look forward to the result. Several hours pass. Under the microscope, six tracks were found. From their position, the half-life was calculated. It turned out to be in the time interval from 0.1 to 0.5 s."
And here is how the same researchers talk about the assessment of the chemical nature of kurchatovium and nilsborium. "The scheme for studying the chemical properties of element No. 104 is as follows. The recoil atoms exit the target into a nitrogen jet, are decelerated in it, and then chlorinated. Compounds of the 104th element with chlorine easily penetrate through a special filter, but all actinides do not pass. If the 104th belonged to the actinoid series, then it would have been delayed by the filter.However, studies have shown that the 104th element is a chemical analogue of hafnium.This is the most important step towards filling the periodic table with new elements.
Then the chemical properties of the 105th element were studied in Dubna. It turned out that its chlorides are adsorbed on the surface of the tube along which they move from the target at a temperature lower than hafnium chlorides, but higher than niobium chlorides. Only atoms of an element close in chemical properties to tantalum could behave in this way. Look at the periodic table: the chemical analogue of tantalum is element number 105! Therefore, experiments on adsorption on the surface of atoms of the 105th element confirmed that its properties coincide with those predicted on the basis of the periodic system.
d-ELEMENTS AND THEIR COMPOUNDS
1. General characteristics of d-elements
The d-block includes 32 elements of the periodic system. d-Elements are included in the 4th-7th major periods. The atoms of the IIIB group have the first electron in the d-orbital. In subsequent B-groups, the d-sublevel is filled up to 10 electrons (hence the name d-elements). The structure of the outer electron shells of d-block atoms is described by the general formula (n-1)d a ns b , where a = 1-10, b = 1-2. A feature of the elements of these periods is a disproportionately slow increase in the atomic radius with an increase in the number of electrons. Such a relatively slow change in the radii is explained by the so-called lanthanide contraction due to the penetration of ns-electrons under the d-electron layer. As a result, there is a slight change in the atomic and chemical properties of d-elements with increasing atomic number. The similarity of chemical properties is manifested in the characteristic feature of d-elements to form complex compounds with various ligands. An important property of d-elements is variable valency and, accordingly, a variety of oxidation states. This feature is mainly associated with the incompleteness of the pre-external d-electron layer (except for the elements of the IB- and IIB-groups). The possibility of the existence of d-elements in different oxidation states determines a wide range of redox properties of elements. In lower oxidation states, d-elements exhibit the properties of metals. With an increase in the atomic number in groups B, the metallic properties naturally decrease. In solutions, oxygen-containing anions of d-elements with the highest degree of oxidation exhibit acidic and oxidizing properties. Cationic forms of lower oxidation states are characterized by basic and reducing properties. d-elements in an intermediate oxidation state exhibit amphoteric properties. These patterns can be considered using the example of molybdenum compounds: With a change in properties, the color of molybdenum complexes in various oxidation states (VI - II) changes: In the period with an increase in the charge of the nucleus, a decrease in the stability of compounds of elements in higher oxidation states is observed. In parallel, the redox potentials of these compounds increase. The greatest oxidizing ability is observed in ferrate ions and permanganate ions. It should be noted that for d-elements, with an increase in relative electronegativity, acidic and non-metallic properties increase. With an increase in the stability of compounds when moving from top to bottom in B-groups, their oxidizing properties simultaneously decrease. It can be assumed that in the course of biological evolution, compounds of elements in intermediate oxidation states, which are characterized by mild redox properties, were selected. The advantages of such selection are obvious: they contribute to the smooth flow of biochemical reactions. A decrease in the RH potential creates the prerequisites for a finer "regulation" of biological processes, which ensures energy gain. The functioning of the body becomes less energy-intensive, and therefore more economical in terms of food consumption. From the point of view of evolution, the existence of d-elements in lower oxidation states becomes justified for the organism. It is known that Mn ions 2+, Fe 2+, Co 2+under physiological conditions, they are not strong reducing agents, and Cu ions 2+and Fe 2+practically do not show restorative properties in the body. An additional decrease in reactivity occurs when these ions interact with bioorganic ligands. It may seem that the important role of the bioorganic complexes of molybdenum(V) and (VI) in various organisms contradicts the above. However, this is consistent with the general pattern. Despite the highest degree of oxidation, such compounds exhibit weak oxidizing properties. It should be noted the high complexing abilities of d-elements, which are usually significantly higher than those of s- and p-elements. This is primarily due to the ability of d-elements to be both donors and acceptors of a pair of electrons that form a coordination compound. In the case of the chromium hydroxocomplex [Cr(OH) 6]3-the metal ion is an electron pair acceptor. Hybridization 3d 24sp 3-orbitals of chromium provides a more stable energy state than when the electrons of chromium are located on the orbitals of hydroxo groups. Compound [CrCl 4]2-is formed, on the contrary, as a result of the fact that the unshared d-electrons of the metal occupy the free d-orbitals of the ligands, since in this case the energy of these orbitals is lower. Properties of the Cr cation 3+show the inconstancy of the coordination numbers of d-elements. Most often, these are even numbers from 4 to 8, the numbers 10 and 12 are less common. It should be noted that there are not only single-core complexes. Numerous di-, tri- and tetra-nuclear coordination compounds of d-elements are known. An example is the binuclear complex of cobalt [Co 2(NH 3)10(O 2)](NO 3)5, which can serve as a model for the oxygen carrier. More than 1/3 of all trace elements of the body are d-elements. In organisms, they exist in the form of complex compounds or hydrated ions with an average hydration shell exchange time of 10 -1to 10 -10With. Therefore, it can be argued that "free" metal ions do not exist in the body: they are either their hydrates or hydrolysis products. In biochemical reactions, d-elements most often manifest themselves as complexing metals. Ligands in this case are biologically active substances, as a rule, of an organic nature or anions of inorganic acids. Protein molecules form bioinorganic complexes with d-elements - clusters or bioclusters. The metal ion (metal-complexing agent) is located inside the cluster cavity, interacting with the electronegative atoms of protein binding groups: hydroxyl (-OH), sulfhydryl (-SH), carboxyl (-COOH) and amino groups of proteins (H 2N -). For a metal ion to penetrate into a cluster cavity, the ion diameter must be commensurate with the cavity size. Thus, nature regulates the formation of bioclusters with d-element ions of certain sizes. The most famous metalloenzymes: carbonic anhydrase, xanthine oxidase, succinate dehydrogenase, cytochromes, rubredoxin. They are bioclusters, the cavities of which form the binding centers of substrates with metal ions. Bioclusters (protein complexes) perform various functions. Transport protein complexes deliver oxygen and necessary elements to the organs. The metal is coordinated through the oxygen of the carboxyl groups and the nitrogen of the amino groups of the protein. This forms a stable chelate compound. The d-elements (cobalt, nickel, iron) act as the coordinating metal. An example of an iron-containing transport protein complex is transferrin. Other bioclusters can play an accumulative (accumulative) role - these are iron-containing proteins: hemoglobin, myoglobin, ferritin. They will be considered in the description of the properties of group VIIIB. The elements Zn, Fe, Co, Mo, Cu are vital, they are part of metalloenzymes. They catalyze reactions that can be divided into three groups: Fe 3+→ Fe 2++ e -
3.Oxygen transfer. Fe, Cu participate. Iron is part of hemoglobin, copper is part of hemocyanin. It is assumed that these elements bind with oxygen, but are not oxidized by it. The d-element compounds selectively absorb light of different wavelengths. This results in coloration. The quantum theory explains the selectivity of absorption by the splitting of the d-sublevels of metal ions under the action of the ligand field. The following color reactions to d-elements are well known: Mn 2++S 2-\u003d MnS ↓ (flesh-colored precipitate) Hg 2++ 2I -= HgI 2↓(yellow or red precipitate) To 2Cr 2O 7+ H 2SO 4(conc.) = K 2SO 4+ H 2O + 2CrO 3↓
(orange crystals) The above reactions are used in analytical chemistry for the qualitative determination of the corresponding ions. The equation for the reaction with dichromate shows what happens when you prepare a "chromium mixture" for washing chemical dishes. This mixture is necessary to remove both inorganic and organic deposits from the surface of chemical vials. For example, greasy contaminants that always remain on the glass after touching fingers. It is necessary to pay attention to the fact that d-elements in the body provide the launch of most biochemical processes that ensure normal life. General characteristics of the d-elements of the VIB group The VIB group consists of elements (transition metals) - chromium, molybdenum and tungsten. These rare metals are found in nature in small quantities. However, due to a number of useful chemical and physical properties, they are widely used not only in mechanical engineering and chemical technology, but also in medical practice (Cr-Co-Mo alloy is used in surgery and dentistry, molybdenum and its alloys are used as parts of X-ray tubes, tungsten make anodes for X-ray tubes, tungsten alloys - the basis of screens for protection against γ -rays). Configuration of valence electrons Cr and Mo - (n-1) d 5ns 1, W - 5d 46s 2. The sum of the valence electrons of chromium, molybdenum, tungsten is 6, which determines their position in the VIB group. For Cr and Mo, the last electron layer is occupied by 13 electrons, for W - 12. Like most d-elements, this layer is unstable. Therefore, the valency of chromium, molybdenum and tungsten is not constant. For the same reason, group VIB metal compounds are characterized by a set of oxidation states from +2 to +6. In the group of d-elements, a general trend is manifested: with an increase in the serial number, the stability of compounds with the highest oxidation state increases. The strongest oxidizing agent in the E state 6+is chromium. "Borderline" Mo 6+exhibits weak oxidizing properties. Molybdenum-on-ion MoO 42-recovers only to Mo 6O 17("molybdenum blue"), where some of the molybdenum atoms have an oxidation state of +5. This reaction is used in analytical chemistry for photometric determinations. In the lower valence states, following the same trend, Cr exhibits stronger reducing properties. 2+. Mo ions 2+and W 2+an increase in the ionization energy leads to a decrease in the reducing and metallic properties. Complex compounds of this group of elements most often have a coordination number of 6 and hybridization of the sp type 3d 2, which in space is described by an octahedron. A characteristic feature of the compounds of this group is the tendency to polymerization (condensation) of the oxygen forms of the elements of group VI. This property is enhanced when moving down the group from top to bottom. In this case, compounds of type M are formed. 6O 2412-, composed of MoO octahedra 4and WO 4. These octahedra form polymer crystals. In chromium (VI) oxide, the ability to polymerize is manifested, but weakly. Therefore, the degree of polymerization is higher for molybdenum and tungsten oxides. According to the structure of the electron shell of atoms with an unfilled d-orbital, the combination of physical and chemical properties, and the tendency to form electropositive ions and coordination compounds, elements of group VI belong to transition metals. Chemical properties of chromium compounds. Most chromium compounds have a bright color in a variety of colors. The name comes from the Greek. chromoc - color, coloration. Compounds of trivalent chromium (unlike molybdenum compounds, and for tungsten the oxidation state +3 is not typical at all) are chemically inert. In nature, chromium is in the trivalent (spinel - double oxide MnCrO 4- magnochromite) and hexavalent state (PbCrO 4- crocoite). Forms basic, amphoteric and acidic oxides. Chromium oxide (II) CrO - red (red-brown) crystals or black pyrophoric powder, insoluble in water. Corresponds to Cr(OH) hydroxide 2. The hydroxide is yellow (wet) or brown. When calcined in air, it turns into Cr 2O 3(Green colour): Cr(OH) 2+ 0.5O 2= Сr 2O 3+ 2H 2O Cation Cr 2+- colorless, its anhydrous salts are white, and water ones are blue. Divalent chromium salts are energetic reducing agents. An aqueous solution of chromium(II) chloride is used in gas analysis for the quantitative absorption of oxygen: 2CrCl 2+ 2НgО + 3Н 2O + 0.5O 2= 2HgCl 2+ 2Cr(OH) 3↓
(dirty green sediment) Chromium(III) hydroxide has amphoteric properties. Easily passes into a colloidal state. Dissolving in acids and alkalis, it forms aqua or hydroxo complexes: Cr(OH) 3+ 3H 3O += [Cr(H 2O) 6]3+(blue-violet solution) Cr(OH) 3+ 3OH -= [Cr(OH) 6]3-(emerald green solution) Compounds of trivalent chromium, like divalent chromium, exhibit reducing properties: Cr 2(SO 4)s + KClO 3+ 10KOH = 2K 2CrO 4 + 3K 2SO 4 + KCl + 5H 2O Chromium(VI) compounds are usually oxygen-containing chromium complexes. Hexavalent chromium oxide corresponds to chromic acids. Chromic acids are formed by dissolving CrO in water. 3. These are highly toxic yellow, orange and red solutions with oxidizing properties. CrO 3forms polychromic acids of composition H 2Cr n O (3n+1) : nCrO 3+ H 2O → H 2Cr n O (3n+1) . There can be several such connections: N 2CrO 4, N 2Cr 2O 7, N 2