Hydrogen halide

diatomic inorganic compound with the formula HX where X is one of the halogens: fluorine, chlorine, bromine, iodine, or astatine

Hydrogen halides (or hydrohalic acids) are inorganic compounds that contain a hydrogen ion and a halide ion. The halogens are fluorine, chlorine, bromine and iodine. Astatine does not make a stable hydrogen halide, so it is not included.

The hydrogen halides are diatomic molecules with no tendency to ionize in the gas phase. Chemists therefore distinguish hydrogen chloride from hydrochloric acid. The former is a gas at room temperature that reacts with water to give the acid. Once the acid has formed, the diatomic molecule can not easily be regenerated.

compound formula structure model d(H−X) / pm
(gas phase)
μ / D Notes
hydrogen fluoride HF 91.7 1.86 Highly toxic, weak acid
hydrogen chloride HCl 127.4 1.11 Strong acid, most common
hydrogen bromide HBr 141.4 0.788 Strong acid
hydrogen iodide HI 160.9 0.382 Strong acid, reducing agent