Triple Bonds: Shared Pairs, Sigma and Pi Bonds, and Bond Order

A triple bond counts three shared pairs

In a Lewis diagram, one line stands for a shared pair of electrons. Three lines between the same two atoms therefore represent six bonding electrons. The bond order of that localized connection is three, even though there are only two connected atoms.

Consider ethyne, C₂H₂, whose skeleton is H–C≡C–H. Its electron budget is 2(4) + 2(1) = 10. Two C–H single bonds consume four electrons; the C≡C triple bond consumes the remaining six. Neither carbon needs a lone pair. Counting the shared electrons around either carbon gives an octet.

A triple bond contains different kinds of overlap

The first connection along an internuclear axis is a σ bond. Two additional, mutually perpendicular side-on overlaps provide two π bonds. Thus a triple bond is one σ plus two π, not three σ bonds. In the simple valence-bond picture of ethyne, each carbon uses sp orbitals for its two σ bonds and retains two unhybridized p orbitals for π overlap.

The two bonding directions at each carbon are linear. Three lines to the same neighbor do not count as three separate directions when considering molecular shape.

Keep three counts separate

A molecule's total-electron audit counts a shared electron only once. An atom's octet check includes the bonding electrons around that atom. Mixing these two operations can falsely suggest that a correct structure contains too many electrons.

Limits of the drawing

Lewis bond order is useful bookkeeping for localized bonds. Delocalized molecules and ions may require several resonance contributors rather than a single integer assignment to every connection. Bond lengths and bond energies are separate physical quantities; a line count alone does not supply their measured values.