Illustrated Glossary of Organic Chemistry

Isoelectronic: Atoms, ions, or molecules which have an equal number of valence electrons and the same atom connectivity.

Carbon monoxide (CO) and dinitrogen (N2) are isoelectronic. Both have ten valence electrons.

Ammonia (NH3) and phosphine (PH3) are isoelectronic. The nitrogen and phosphorus atoms of these molecules are also isoelectronic.