We use bond valence (BV) theory to rationalize why certain non-protonic cations preferentially occupy certain crystallographic sites. This relatively simple electrostatic model of crystal chemistry modifies Pauling's second rule by relating the strength of a cation-anion bond (s i ) to its length (R i ) via two bond-specific empirically fitted constants (R o & b). Summing the strengths of the bonds to a cation (details in Table ) as defined by Eq. 7,