<P> A solid compound containing metals can also be an insulator if the valence electrons of the metal atoms are used to form ionic bonds . For example, although elemental sodium is a metal, solid sodium chloride is an insulator, because the valence electron of sodium is transferred to chlorine to form an ionic bond, and thus that electron cannot be moved easily . </P> <P> A semiconductor has an electrical conductivity that is intermediate between that of a metal and that of a nonmetal; a semiconductor also differs from a metal in that a semiconductor's conductivity increases with temperature . The typical elemental semiconductors are silicon and germanium, each atom of which has four valence electrons . The properties of semiconductors are best explained using band theory, as a consequence of a small energy gap between a valence band (which contains the valence electrons at absolute zero) and a conduction band (to which valence electrons are excited by thermal energy). </P>

State the number of valence electrons in an atom of sulfur