The chemical formula for the bromine gas is Br2. This signifies that the bromine molecule contains only two "Br" atoms, which form a covalent bond between them.
The chemical formula of a substance does not depend on its state of matter. Bromine is typically a reddish brown liquid at standard temperature and pressure, but it easily evaporates into an amber-colored gas, even at normal room temperature. Its chemical properties are similar to other halogens such as chlorine and iodine. Human exposure to bromine gas is typically fatal, which is why special care should be taken when manipulating this highly toxic substance.