A colorless, highly toxic gas having the molecular formula C₂N₂. Cyanogen belongs to a class of compounds known as pseudohalogens, because of the similarity of their chemical behavior to that of the halogens. Liquid cyanogen boils at −21.17°C (−6.11°F) and freezes at −27.9°C (18.2°F) at 1 atm (10⁵ Pa). The density of the liquid is 0.954 g/ml at the boiling point.