Melting Point Trend For Non Metals?
as one moves along the period, the atomic radii decrease. You see, this decrease leads to an increased net Z-effective and hence, the electrons come very close. In the case of Nitrogen, which exists as a diatomic molecule - this shrinking in size is less than that for oxygen and fluorine. 
    the melting point is determined by the intermolecular interactions as well as the geometry of molecules. greater the number of bonds to break, higher is the melting point. For oxygen and especially Fluorine, there can be a forced compression, but yet again - these small atoms repel each other because of their high electron density. This repulsion lowers the melting and boiling points.

As for noble gases like Argon, they do not need to form any sorts of bond. The only kind of forces acing between two atoms are very weak London forces. These London forces are responsible for the freezing of Helium, at temperature as low as -273K, very close to absolute zero!!  

I hope that helps!!