Why does Phosphorus have a higher melting point than Chlorine?

Phosphorus exists as P₄ and chlorine as Cl₂ so elemental phosphorus is a bigger molecule than chlorine. Neither of these molecules has a dipole so both have only induced dipole-dipole (or Van Der Waal's) forces between each molecule. This is due to the random movement of electrons in the molecule creating instantaneous dipoles in each molecule which therefore induces instantaneous dipoles in the neighbouring molecules. The interaction between the dipoles of these molecules (an intermolecular force) stabilises the solid form of the molecule. P₄ is bigger than Cl₂ and it has more electrons therefore stronger Van Der Waal's forces so a higher melting point as more energy is needed to overcome these intermolecular forces.