The original motivation for introducing the concept of atomicity is as follows: It may seem natural to define covalency as |C| 2 /(|C| 2 + |I| 2 ) and ionicity as |I| 2 /(|C| 2 + |I| 2 ). However, such a choice leads directly to unphysical features (see Fig. in Supporting Information). Namely, |C| 2 /(|C| 2 + |I| 2 ) reaches its maximal value of unity in the limit of separate atoms (R → ∞). This is also the limit when electrons are entirely localized on their parent atoms and become distinguishable in the quantum mechanical sense. This is the limit which we regard as that atomicity equal to unity and vanishing true covalency. This type of argument is also the reason of subtracting the probability Ã2 from C2 , not the corresponding wave-function amplitudes. The quantities ( )- (10) are proved next to be useful and of crucial importance.