Covalent Bonds

Every element except the noble gases occurs in nature either as a compound or as a polyatomic/diatomic molecule -- never as isolated, uncombined atoms. Consider hydrogen gas: two hydrogen atoms bind together to give the diatomic molecule H₂. Each hydrogen atom has one electron, and needs one MORE electron to reach helium's stable duet configuration. Lewis's insight was that both hydrogen atoms can attain this stable configuration simultaneously by mutually SHARING the one electron each already has, so that both atoms 'see' two electrons.

The same logic scales up: in an oxygen molecule, the two oxygen atoms share TWO electron pairs between them; in a nitrogen molecule, THREE electron pairs are shared between the two nitrogen atoms. This mutual sharing of one or more pairs of electrons between two combining atoms is what defines a covalent bond.

• If two atoms share one pair of electrons, the result is a single covalent bond (e.g. H₂, H-H). …