Fluorine (F2) has a lower boiling point than chlorine (Cl2). The boiling points of these diatomic molecules can be primarily understood through the concept of molecular size and intermolecular forces.
Fluorine is a smaller molecule compared to chlorine. As a result, F2 experiences weaker London dispersion forces, which are the main type of intermolecular forces present in nonpolar molecules like these. These forces arise due to temporary dipoles that occur when electrons move around the nucleus.
Because chlorine molecules are larger and have more electrons, they exhibit stronger London dispersion forces. This results in more energy being required to separate the molecules during the phase change from liquid to gas, which in turn raises the boiling point.
To summarize, the smaller size of F2 leads to weaker intermolecular forces when compared to the larger Cl2, thus giving fluorine a lower boiling point.