Isostere

Classical Isosteres are molecules or ions with the same number of atoms and/or the same number of valence electrons. The definition was later revised to include compounds with similarly reactive electron shells. For example, Hydrogen ion and Fluoride are classical isosteres because they both have relatively small nuclei and their outer valence shells are full (or empty in Hydrogen's case) when ionized (0 in Hydrogen, 1s + 2p = p in Fluoride).