Cadmium interatomic distance

In other crystals an octahedral metal atom is attached to six non-metal atoms, each of which forms one, two, or three, rather than four, bonds with other atoms. The interatomic distance in such a crystal should be equal to the sum of the octahedral radius of the metal atom and the normal-valence radius (Table VI) of the non-metal atom. This is found to be true for many crystals with the potassium chlorostannate (H 61) and cadmium iodide (C 6) structures (Table XIB). Data are included in Table XIC for crystals in which a tetrahedral atom is bonded to a non-metal atom with two or three covalent bonds. The values of dcalc are obtained by adding the tetrahedral radius for the former to the normal-valence radius for the latter atom. 

For cadmium the observed interatomic distances 2-973 (6) and 3-287 A (6) indicate bond numbers and , respectively, as for zinc, and lead to the values 1-397 and 1-410A fori , in good agreement with the value 1-400A given in the table for v = 4. Bond number for the longer bonds would lead to Rt = 1-463, in disagreement with the value 1- 384 for v = 4 . Hence we conclude that in the elementary metal cadmium, like zinc, is quadrivalent. 

Mercury, with interatomic distances 2-999(6) and 3-463(6), appears to have valency 3 . With bond numbers and, respectively, these distances lead to Rx = 1-410 and 1-498, the latter being much too large fort = 4(1 = 1-403), whereas bond numbers and lead to I i = 1-410 and 1-408, in approximate agreement with the value 1-418 for v — 3 . The decrease in valency from cadmium to mercury conforms to a general trend toward smaller metallic valencies with increasing atomic number in a group of elements. 

From the observed values of interatomic distances in complex ions such as [SnCh]—, [PbBr0], and [SeBr ]— and from crystals such as TiS2 with the cadmium iodide structure the octahedral radii given in Table 7-17 have been obtained. These correspond not to cPsp bonds, involving d orbitals of the shell within the valence shell, but to sp d2 orbitals, use being made of the unstable d orbitals of the valence shell itself. 

V3 = 1.633 that makes the twelve smallest interatomic distances egual the two exceptional substances, zinc and cadmium, are discussed below, together with several other metals that crystallize with structures obtained from closest-packed arrangements by a deformation that shortens some of the twelve small interatomic distances at the expense of others. 

In such cases the mean of the two distances is taken. (We are not here referring to zinc and cadmium which show a very much larger deviation from closest packing, with axial ratios 1-856 and 1 885 respectively.) For metals which crystallize with structures of lower coordination the radii for 12-coordination have to be derived in other ways. From a study of the interatomic distances in many metals and alloys Goldschmidt found that the apparent radius of a metal atom varies with the coordination number in the following way. The relative radii for different... 

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