Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in the structure.

For example, CO, NO⁺
, and N
₂ are isoelectronic, while CH
₃COCH
₃ and CH
₃N=NCH
₃ are not.

This definition is sometimes termed valence isoelectronicity. Definitions can sometimes be not as strict, sometimes requiring identity of the total electron count and with it the entire electronic configuration. More usually, definitions 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, so identifying a compound as isoelectronic with one already characterised offers clues to possible properties and reactions (Differences in properties such as electronegativity of the atoms in isolelectronic species can affect reactivity.)

In quantum mechanics, hydrogen-like atoms are ions with only one electron such as Li²⁺
. These ions would be described as being isoelectronic with hydrogen.