Bromine (Br), an element and member of the halogens (Group 17 /Group VIIa) of the periodic table, is the only nonmetal that exists in liquid form at room temperature.
The molecules of each substance are held together by intermolecular forces. In case of diatomic molecules of non-metals e.g. Br2, Cl2, H2 the responsible intermolecular forces are weak (London) dispersion forces.
The state in which a substance exists depends on the kinetic energies of the molecules and their intermolecular attractions and these attractive forces increase from top to bottom in a Group. Fluorine is on the top of the Group; thus, the atomic radius is very small and the electrons are tightly held to the nuclei. The electrons cannot easily gather to one side of the molecule to form a temporary dipole, so the London dispersion forces are relatively weak. As we move down the group, the electrons move further away from the nuclei so the electron clouds can more easily distort, forming stronger intermolecular forces. Hence, Fluorine and Chlorine are gases at room temperature, Bromine is a liquid and Iodine exists as a Solid.
Bromine is one of only two elements (the other being Mercury) that is liquid at room temperature.