# Melting and boiling point comparison

Graphite, silica and diamond are covalent compounds and still they have high melting and boiling points. Why?

On the other hand, we have $\ce{N2}$, $\ce{O2}$, and other covalent molecules. They may be quite strong (in that each molecule by itself is pretty hard to tear apart), but the molecules are linked together by the so-called van der Waals forces, and these are quite weak, so these compounds are gases, and it takes some effort to see them turning into a liquid or a solid.