Explain the following trend in melting temperatures for the halogens.?

Halogens interact using Van Der Waal forces. As you go down a group the relative atomic size increases and as these increase so do the relativly weak Van Der Waal forces. The more electrons a halogen has the more often it can create a dipole moment meaning the stronger the bond will be. Check out my reference below for more info. Oh and they've left the one out of 17 and so have refered to them as group 7 elements.