Even metallic sheen can be explained by structure. A glass surface or a mirror appears bright because the light that falls on it is reflected efficiently.

Mirrors are simply glass, with a thin metal layer as a backing. When light strikes a metal, it is absorbed and the light energy raises electrons to excited (higher) energy states.

There are many mobile electrons in a metal, and many closely spaced energy levels. The excited electrons can move about, and can drop back to their original low-energy states, giving back the energy as photons of light.

Non-metals lack these closely spaced electronic energy states, so light, once absorbed, is less likely to be reemitted.

Melting points are useful measures of the forces between molecules or ions. Molecular solids, in which covalently bonded molecules are packed together with nothing but van der Waals forces between them, have low melting points.

Solid , for example, melts at -218°C. Lithium metal, which has mobile electrons holding it together, melts at 179°C, and LiF, a typical salt, melts at a much higher 842°C.

Solids that are held together completely by covalent bonds in a three-dimensional. network are the most tightly knit of all, and carbon in the form of diamond has a melting point of over
3600°C.