Isoelectronicity

Not to be confused with Isoelectric Point/Focusing or the baseline of the Electrocardiogram the Isoelectric Line

Isoelectronicity is the phenomenon of two or more chemical species (atoms, molecules, radicals, ions etc.) differing in the atoms that comprise them but having the same number of valence electrons and the same structure (that is, the same number of atoms with the same connectivity). The species concerned are termed isoelectronic.

This definition is sometimes termed valence isoelectronicity, in contrast with various alternatives. At one extreme these require identity of the total electron count and with it the entire electron configuration. More usually, alternatives are broader, and may extend to allowing different numbers of atoms in the species being compared.

The importance of the concept lies in identifying significantly related species, as pairs or series. Isoelectronic species can be expected to show useful consistency and predictability in their properties. (Slight differences of, for example, structural formula, such as a double versus single bond, commonly have major effects.)

Electron-density calculations have been performed on many common substances, resulting in reaction predictions. Identifying a new, rare or odd compound as isoelectronic with one already characterised offers clues to possible properties and reactions.

The N atom and the O+
radical ion are isoelectronic because each has five electrons in the outer electronic shell. Similarly, the cations K+
, Ca2+
, and Sc3+
and the anions Cl−
, S2−
, and P3−
are all isoelectronic with the Ar atom. In such monatomic cases, there is a clear trend in the sizes of such species, with atomic radius decreasing as charge increases.

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