Which has a lower boiling point chlorine or bromine, and why?

Chlorine has a lower boiling point as it is smaller in size than bromine. Chlorine has fewer electrons than bromine and fewer electron shells so chlorine is smaller in atomic radius. Size affects the strength of the intermolecular forces and as the size of a molecule decreases the less energy is needed to break the forces so the lower the boiling point. As chlorine is smaller and has weaker intermolecular forces than bromine, less heat energy is required to break the bonds.