Chlorous acid is a chemical compound with the formula HClO2. It is a weak acid. The pure substance is unstable, degrading to chloride, chlorate and chlorine dioxide. But chlorite salts such as sodium chlorite are stable conjugate bases derived from this acid. These salts are sometimes used in the production of chlorine dioxide.
Chlorine is the only one of the four halogens to form an isolable acid of formula HXO2. Fluorine does not have the bonding capacity to do so, whereas the hypothetical bromous acid and iodous acid have never been isolated, nor have any of their salts.