Ethyne

In ethyne (acetylene) each carbon bonds to only two atoms: one hydrogen and the other carbon. Each carbon mixes its 2s orbital with one 2p orbital to make two sp hybrid orbitals, which point in exactly opposite directions. So the whole molecule is a straight line: every angle is 180°. Turn on Angles & lengths to check.

Each carbon still has two unused p orbitals, perpendicular to the molecule’s axis and to each other. Turn on p orbitals: you’ll see two sets of lobes on each carbon, one pair pointing up and down and one pair pointing toward and away from you. Each set overlaps sideways with its partner on the other carbon, making two π bonds at right angles. A triple bond is one σ bond plus two π bonds.

Turn on π electrons and rotate the model. The two π bonds blend into a single tube of electron density wrapped around the C–C axis. With three bonds holding them together, the carbons sit very close: the C≡C bond is about 1.20 Å, shorter than ethene’s double bond (about 1.33 Å) and ethane’s single bond (about 1.53 Å).