A compound is a pure substance that contains two or more elements chemically combined in fixed proportions by mass. Bromine gas consists of two bromine atoms chemically bonded together to form a diatomic molecule with the formula Br2. The atoms in a compound are held together by chemical bonds, and the proportions of the elements in a compound are always the same. Bromine gas is a compound because it consists of two bromine atoms chemically bonded together in a fixed proportion, and it has a definite chemical formula, Br2.