Take the ionic radius of Na⁺ as 0.102 nm and that of Cl⁻ as 0.181 nm. What cation coordination number does the radius ratio predict? Take the coordination number as 4 for a radius ratio of 0.225–0.414, 6 for 0.414–0.732, and 8 for 0.732 and above.

Answer: B. 6

The radius ratio is r₊/r₋ = 0.102/0.181 ≈ 0.564, in the range 0.414–0.732, so octahedral coordination is predicted, with the cation surrounded by 6 anions (coordination number 6). NaCl indeed has the rock salt structure, in which both Na⁺ and Cl⁻ have a coordination number of 6. Coordination number 8 is stable for radius ratios of 0.732 and above, as in CsCl, and coordination number 4 for 0.225–0.414, as in ZnS. The idea behind this rule (Pauling's first rule) is that if the cation is too small, the surrounding anions run into each other and no longer touch the cation, so the coordination number drops.