Group variation of bond distances

Both theM-Cl and the MH bond distances increase monotonically as Group 2 is descended. As one moves along the fourth period from Ca to Zn, the nuclear charge increases by +10 while 10 electrons are filled into the 3d orbitals. We have already seen that this leads to a substantial decrease of the size of the metal atom (t/-block contraction). Comparison of M-Cl bond distances in the dichlorides or M-H bond distances in the monohydrides 

On going from Sr to Cd in the fifth-period, the nuclear charge again increases by 10 units while 10 electrons are filled info fhe Ad orbitals. At the same time the bonding radius of the metal appears to decrease by about 36 pm, and Cd atom is smaller than both the Sr and Ca atoms. 

The mean M-C bond energies in M(CH3)2, M = Zn, Cd, or Hg, defined as half the standard energies of the reactions 

Structure and bonding in the gaseous monochlorides, trichlorides and trimethyl derivatives of the Group 13 elements 

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Fig. 11.1. Group variation of bond distances in the subvalent Group 13 element chlorides, MCl(g), in the Lewis-valent trichlorides, MCl3(g) and in the trimethyl derivatives M(CH3)3.

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