I noticed that the melting point for non metals decreases across a period (i.e. melting point of nitrogen is higher than oxygen, oxygen's is higher than fluorine, so on). It is significantly more noticeable for chlorine and argon. Why is this? Furthermore, I a question came up in my homework asking sulfur (S8) had a higher melting point than phosphorus (P4). Their explanation was that sulfur is a larger molecule, but I don't see how this affects melting point. Is there a better explanation?
Note - this is the graph I was referring to for melting point: http://antoine.frostburg.edu/chem/senese/101/periodic/images/meltingpoint.gif