isoelectronic principle

Why do carbon dioxide CO2, nitrous oxide N2O and the azide ion N3- all turn out to be straight, linear molecules with similar bonding? Because they have the same number of electrons. The isoelectronic principle says that species with the same number of valence electrons (and the same number of atoms) tend to share the same structure, shape and bonding — so once you understand one, you understand the whole family.

Two species are isoelectronic if they have the same total count of valence electrons arranged among the same number of atoms. The classic trio CO2, N2O and N3- each has 16 valence electrons spread over three atoms, so all three are linear with two double-bond-like linkages. Likewise N2, CO and the cyanide ion CN- are all 10-valence-electron diatomics with a triple bond — which is exactly why CO and CN- are such good metal ligands, mimicking the bonding of nitrogen. Add or remove an atom plus the matching electrons and you hop to the next member of an isoelectronic series.

The principle is a powerful shortcut and a generator of predictions in inorganic chemistry. It lets you guess the shape and reactivity of an unfamiliar species by mapping it onto a familiar one, it underpins the family resemblances of the oxoanions, and it inspires materials design — boron nitride BN is isoelectronic with carbon C2 and indeed forms both a graphite-like sheet and a diamond-like hard solid. It is an empirical regularity, not an exact law: isoelectronic species are similar, but differences in nuclear charge still make CO polar where N2 is not, and shift bond lengths and energies.