Isoelectronicity

Two or more molecular entities (atoms, molecules, or ions) are described as being isoelectronic with each other if they have the same number of electrons or a similar electron configurationand the same structure (number and connectivity of atoms), regardless of the nature of the elements involved.

The term valence isoelectronic is used when these molecular entities have the same number of valence electrons or a similar electron configuration, but may have a different number of atoms or a different bonding.

The statement "These compounds or molecules are isoelectronic" is not just an implementation of the above definition. It has significance by the fact that calculations on molecules and electron density, and therefore capability of reaction, have been performed on many common substances. Identifying a new, rare or odd compound as being isoelectronic with an already known one offers clues to possible properties and reactions.

Examples

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.