The reason chlorine is a gas while bromine is a liquid at room temperature is because they contain different intermolecular forces.

Room temperature is usually taken as being 25°C. At this temperature, fluorine and chlorine are gases, bromine is a liquid, and iodide and astatine are solids. There is therefore a trend in state from gas to liquid to solid the group. The reason why chlorine is a gas while bromine is a liquid at room temperature is because they contain different intermolecular forces. The attractive forces between the molecules of these elements increases from top to the bottom of the Group. Chlorine has weaker forces of attraction compared to bromine and that is why it is a gas at room temperature.