Why do metals become insulators when oxidized? I don't know the connection between how forming a new bond with oxygen then changes the density of states to transform the metal into an insulator. It seems like a very powerful transformation.
 A: Metals are good conductors of electricity because the outer (valence) electrons of the metal atoms are only loosely bound to the nucleus and form molecular orbitals known as the conduction band. Electrons can move more or less freely through the conduction band and so metals conduct electricity generally well.
When a metal is chemically oxidised its outer electrons are removed in an oxidation reaction we can write as, for a generic metal $\mathrm{M}$:
$\mathrm{M} \to \mathrm{M^{z+}} + ze^-$
The electrons become absorbed by oxygen molecules:
$\frac{z}{4}[\mathrm{O_2}+4e^- \to 2\mathrm{O^{2-}}]$
And the metal cations and oxide anions combine to form a metal oxide (ionic) lattice:
$\mathrm{M^{z+}}+\frac{z}{2}\mathrm{O^{2-}} \to \mathrm{MO_{z/2}}$
But since as the metal atoms now have been stripped of their outer (valence) electrons, this ionic lattice cannot conduct electricity appreciably.
A: It's because valence electrons are bounded. For example, consider Si and SiO2. While Si is semiconductor, SiO2 is insulator because it has no free valence electrons. BTW, many metal oxides ARE NOT in fact insulators - for example ZnO, Fe3O4 are all conductors. But it's true that oxides of metals have lower conductivity than pure metals.
