Calculation of the M-X bond distance in crystalline alkali metal halides

11 Calculation of the M-X bond distance in crystalline alkali metal halides 

All the alkali metal halides except CsCl, CsBr and Csl form cubic crystals with rock-salt structure, where each metal cation is surrounded by six halide anions, and each anion surrounded by six cations at the comers of an octahedron. See Fig. 5.3. Consultation of a textbook on elementary inorganic or physical chemistry should make it clear that the energy of one mole of the crystalline material relative to the separated, neutral atoms is given by 

In order to determine the bond distances in the crystalline phase we proceed in the same fashion as for the dimers Using the fact that the derivative of YdR) with respect to R must be zero when R is equal to the equilibrium bond distance which we shall denote by Rc, we 

In Table 5.2 we summarize the result of our calculations of bond distances. We see that M-X bond distances increase with increasing aggregation, i.e. with increasing coordination numbers. This elongation is due to Coulomb repulsion between the increasing number of anions around each cation (and to repulsion between the increasing number of cations around each anion). 

GASEOUS ALKALI METAL HALIDES IONIC BONDS 

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