Under extreme conditions of heat, diamond could be vaporized. Because diamond is a non-molecular material that is held together by strong C-C bonds, conditions would have to be exceptionally fierce.
Given that C(g) would be much less dense than C(s), we could not accurately depict diamond atoms, carbon atoms, on the same scale.
A simple molecular substance is simply a discrete molecule, i.e. O_2, CO_2, N_2, a material with definite molecularity, i.e. boundaries. Of course, you could choose many others. A giant covalent substance is a non-molecular species, whose particles are individually linked by strong covalent bonds in an extended lattice with no molecular boundaries. Quartz, diamond, graphite, are examples.
As for question 4(b), this is a rather nasty one. MY own feeling is that C_60 is a molecular species, because it contains discrete molecules with definite size and structure. Certainly, an organic chemist can make bigger molecules than this.
