The bond angle in hydrogen cyanide (HCN) is approximately 180 degrees.
This occurs because HCN has a linear molecular geometry. In HCN, the carbon atom is double-bonded to nitrogen and single-bonded to hydrogen. The arrangement of these bonds causes the molecule to take on a straight line shape, which naturally leads to the bond angle of 180 degrees.
Both the carbon-nitrogen and the carbon-hydrogen bonds are in a straight line, minimizing the repulsion between the pairs of bonding electrons. This linear configuration is characteristic of molecules that possess a triple or double bond between two atoms, as it stabilizes the molecule and reduces overall energy.