Question 4
Covalent substances usually melt and boil at low temperatures, yet diamond and graphite do not. Why?
The right answer is the one marked above.
Melting them means breaking covalent bonds throughout a giant lattice. In an ordinary covalent substance, melting only separates small molecules from one another, which takes far less energy.
Neither is ionic — there are no ions in pure carbon. Neither contains molecules at all, so their molecular mass is not the point, and carbon atoms here are held by shared pairs rather than by charge.