<P> In these simple cases where the octet rule is obeyed, the valence of an atom equals the number of electrons gained, lost, or shared in order to form the stable octet . However, there are also many molecules which are exceptions, and for which the valence is less clearly defined . </P> <P> Valence electrons are also responsible for the electrical conductivity of an element; as a result, an element may be classified as a metal, a nonmetal, or a semiconductor (or metalloid). </P> <P> Metallic elements generally have high electrical conductivity when in the solid state . In each row of the 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 . 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 . </P> <P> A nonmetallic element has low electrical conductivity; it acts as an 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 . </P>

What type of bond is most likely for two atoms with valences of 1 and 7