Covalent bonding: sharing electrons

Sharing electrons

When two atoms each need extra electrons to reach the octet (or the duet), and neither really wants to give up its own, they find a compromise: they share one or more pairs of electrons. This is a covalent bond. It typically appears between two non-metal atoms, whose electronegativity is close or identical.

The bonding pair

The shared pair of electrons is called a bonding pair. Each atom generally contributes one electron to this pair, and the two electrons are then counted in the valence shell of both atoms at once. This is an accounting trick: the same electrons "count double".

H2 : H--H            (a single shared pair, one bonding pair)

Single, double or triple bond

Two atoms can share more than one pair of electrons:

• single bond: 1 shared bonding pair (example: H-H),
• double bond: 2 shared bonding pairs (example: O=C=O, in carbon dioxide),
• triple bond: 3 shared bonding pairs (example: N≡N, in dinitrogen).

The more pairs a bond shares, the shorter and stronger it is.

Example: the water molecule (H2O)

Oxygen has 6 valence electrons; it is missing 2 to reach the octet. Each hydrogen has 1 electron; it is missing 1 to reach the duet. Oxygen therefore forms a single covalent bond with each of the two hydrogens: two bonding pairs in total, plus two lone pairs (pairs of oxygen's electrons that are not shared).

Classic mistake

Don't confuse the number of shared electrons with the number of bonds: a double bond is not "two different bonds", but a single chemical bond reinforced by two bonding pairs between the same two atoms.