Editing Valence electron (section)

==Electrical conductivity==
Valence electrons are also responsible for the bonding in the pure chemical elements, and whether their [[electrical conductivity]] is characteristic of metals, semiconductors, or insulators.

{{periodic table (simple substance bonding)}}

[[Metal]]lic elements generally have high [[Electrical conductor|electrical conductivity]] when in the [[solid]] state. In each row of the [[Periodic table (metals and non-metals)|periodic table]], the metals occur to the left of the nonmetals, and thus a metal has fewer possible valence electrons than a nonmetal. However, a valence electron of a metal atom has a small [[ionization energy]], and in the solid-state this valence electron is relatively free to leave one atom in order to associate with another nearby. This situation characterises [[metallic bond]]ing. Such a "free" electron can be moved under the influence of an [[electric field]], and its motion constitutes an [[electric current]]; it is responsible for the electrical conductivity of the metal. [[Copper]], [[aluminium]], [[silver]], and [[gold]] are examples of good conductors.

A [[Nonmetal (chemistry)|nonmetal]]lic element has low electrical conductivity; it acts as an [[insulator (electrical)|insulator]]. Such an element is found toward the right of the periodic table, and it has a valence shell that is at least half full (the exception is [[boron]]). Its ionization energy is large; an electron cannot leave an atom easily when an electric field is applied, and thus such an element can conduct only very small electric currents. Examples of solid elemental insulators are [[diamond]] (an [[allotrope]] of [[carbon]]) and [[sulfur]]. These form covalently bonded structures, either with covalent bonds extending across the whole structure (as in diamond) or with individual covalent molecules weakly attracted to each other by [[intermolecular forces]] (as in sulfur). (The [[noble gas]]es remain as single atoms, but those also experience intermolecular forces of attraction, that become stronger as the group is descended: helium boils at −269&nbsp;°C, while radon boils at −61.7&nbsp;°C.)

A solid compound containing metals can also be an insulator if the valence electrons of the metal atoms are used to form [[ionic bond]]s. 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.

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).