D-block contraction

Hg is much more dense than Cd, because the decrease in atomic radius that occurs between Z = 58 and Z = 71 (the lanthanide contraction) causes the atoms following the rare earths to he smaller than might have been expected for their atomic masses and atomic numbers. Zn and Cd have densities that are not too dissimilar because the radius of Cd is subject only to a smaller d-block contraction. 

Is there evidence for a d-block contraction analogous to the lanthanide contraction Explain your reasoning. 

Two trends are apparent in Table 2.8. Firstly, in going from Sc to to La there is a stepwise increase in radius as principal shells of electrons are added. However, in all of the other groups the increase between the first and second row is not repeated in the second to third row. This is a result of the lanthanide contraction and the filling of the 4f subshell between La and Hf. This reflects the poor screening of 4f electrons one by another, leading to an increase in and a decrease in radius. Secondly, for metals in the same oxidation state, there is a d-block contraction across the rows as a result of the increase in Z... 

Thus, the main relativistic effects are (1) the radical contraction and energetic stabilization of the s and p orbitals which in turn induce the radial expansion and energetic destabilization of the outer d and f orbitals, and (2) the well-known spin-orbit splitting. These effects will be pronounced upon going from As to Sb to Bi. Associated with effect (1), it is interesting to note that the Bi atom has a tendency to form compounds in which Bi is trivalent with the 6s 6p valence configuration. For this tendency of the 6s electron pair to remain formally unoxidized in bismuth compounds (i.e. core-like nature of the 6s electrons), the term inert pair effect or nonhybridization effect has been often used for a reasonable explanation. In this context, the relatively inert 4s pair of the As atom (compared with the 5s pair of Sb) may be ascribed to the stabilization due to the d-block contraction , rather than effect (1) . On the other hand, effect (2) plays an important role in the electronic and spectroscopic properties of atoms and molecules especially in the open-shell states. It not only splits the electronic states but also mixes the states which would not mix in the absence of spin-orbit interaction. As an example, it was calculated that even the ground state ( 2 " ) of Bij is 25% contaminated by Hg. In the Pauli Hamiltonian approximation there is one more relativistic effect called the Dawin term. This will tend to counteract partially the mass-velocity effect. 

The M-C bond distances in the trimethyl derivatives and the M-Cl bond distances in both trichlorides and monochlorides are displayed in Fig. 11.1. The M-C bond distances are seen to increase from B to Al, but to remain essentially constant from A1 to Ga. The unexpected shortness of the Ga-C bond reflects the d-block contraction. The break at Ga is followed by a normal increase from Ga to In and a smaller increase from In to Tl. The unexpected shortness of the Tl-C bond reflects the/-block contraction as well as relativistic effects. 

Arsenic exhibits a marked reluctance to adopt a pentavalent state. For example, unlike PCI5 and SbCls, which are stable and commercially available substances, ASCI5 decomposes at -50 °C. It was first prepared only in 1976 by UV irradiation of ASCI3 in liquid chlorine at -105 °C. The anomalous instability of AsClj is generally ascribed to a phenomenon called d-block contraction, which stabilizes the 4s lone pairs of the elements following the first transition series. Unfortunately, limitations of space do not permit a proper discussion of the phenomenon. 

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