Hydrogen chloride. Composite image of the atomic and electronic structure of the molecule hydrogen chloride. This diatomic molecule consists of a hydrogen atom (H, left) forming a single covalent bond with a chlorine atom (Cl, right). This covalent bond is formed by the sharing of electrons (centre). The single hydrogen electron (pink) combines with the unpaired electron (green) in the chlorine atom's outer electron shell. This fills the outer electron shell, forming a strong and stable bond. Hydrogen chloride is a colourless gas that forms hydrochloric acid when dissolved in water.