When forming a cation, an atom of the main group element tends to lose all of its valence electrons, thus assuming the electronic structure of the noble gas that precedes it in the periodic table. For groups 1 (the alkali metals) and 2 (the alkaline earth metals), the group numbers are equal to the numbers of valence shell electrons and, consequently, to the charges of the cations formed from atoms of these elements when all valence shell electrons removed.

For example, calcium is a group 2 element whose neutral atoms have 20 electrons and a ground-state electron configuration of 1s²2s²2p⁶3s²3p⁶4s². When a Ca atom loses both of its valence electrons, the result is a cation with 18 electrons, a 2⁺ charge, and an electron configuration of 1s²2s²2p⁶3s²3p⁶. The Ca²⁺ ion is, therefore, isoelectronic with the noble gas Ar.