A-Level How atoms bond and how that governs a substance's behaviour: worked solution

Question

Suggest how the electrons in the second shell of an sp carbon atom are arranged so that the bonding between the two carbons of C2H2 can occur.

In ethyne, C2H2, each carbon is sp hybridised. The four second-shell electrons on each carbon are arranged so that two of the four atomic orbitals mix: the 2s orbital and one 2p orbital combine to give two sp hybrid orbitals (arranged linearly, 180 degrees apart). Each carbon therefore has two sp hybrid orbitals plus two unhybridised 2p orbitals, each containing one electron. One sp hybrid on each carbon overlaps end-on to form a C-C sigma bond, and the other sp hybrid overlaps with the hydrogen 1s to form a C-H sigma bond. The two remaining singly-occupied 2p orbitals on each carbon (perpendicular to the C-C axis and to each other) overlap side-on with the matching 2p orbitals of the other carbon to form two pi bonds. This gives the C=C triple bond (one sigma + two pi).