# The chemical formula of acetylene is C_2H_2. What type of bond holds the two carbon atoms together?

A $C \equiv C$ triple bond.
Each carbon contributes 4 electrons; each hydrogen contributes 2 electrons. A Lewis representation must therefore distribute 10 electrons: $2 \times C - H$ bonds, 4 electrons, and $1$ $C \equiv C$ triple bond, 6 electrons.
A valence bond formalism would gives rise to 2 $s p$ hybrid orbitals on each $C$ atom; one such hybrid interacts with $H$ to form a $C - H$ bond, and the remaining orbital forms a $C - C$ bond. The remaining 4 electrons remain unhybridized and forms $\pi$ bonds above and below the plane of the $C - C$ vector.