Consider the covalent bonding in the molecule acetylene [ah-SET-ihl-een], and assume that the carbon-carbon axis is the x-axis. What two orbitals on each carbon atom participate in pi bonding?

2py and 2pz orbitals

Explanation:

Each carbon atom still has two half-filled 2py and 2pz orbitals, which are perpendicular both to each other and to the line formed by the sigma bonds. These two perpendicular pairs of p orbitals form two pi bonds between the carbons, resulting in a triple bond overall (one sigma bond plus two pi bonds).