When you examine the halogens at room temp. you realize that fluorine and chlorine are both gases, bromine is?

As we go down Group 7 of the periodic table, the masses and sizes of the molecules increases. This increases surface area in which the molecules are in contact with each other. With bigger surface area, there is stronger intermolecular forces between them. Hence, overcoming these forces require more energy. Therefore, as we go down the group, the melting and boiling point increases.