An approach is presented to calculate topological coordination numbers (tCNs) obeying the principle of coordination reciprocity from solid angles subtended by the interatomic surfaces of electron density (QTAIM) atomic domains. The tCN approach characterizes a compound's coordination situation as a set of sub-coordination scenarios with associated weights, which is considered a suitable input for future AI applications on structure–property relationships.

In lacunary apatite structure-type Pb4Na(PO4)3, residual density attributable to the deformed 6s2 electron cloud is found in the vicinity of A2-site Pb2+ on regular A2 triangle normal to c. Systematic comparison on the size of the A2 triangle with other apatite-type compounds indicates limited stereochemical activity of the orbital.

Ion radii are derived from observed mean bond lengths (characteristic bond lengths) for 135 ions bonded to oxygen, fluorine, hydroxyl, chlorine and nitro­gen. Radii derived from quantum-mechanical calculations do not agree with radii derived from experimentally determined bond lengths. However, this problem is removed by recognition that ion radii determined from characteristic bond lengths are not a measure of the sizes of ions but proxy variables for characteristic bond lengths.