Number of bonds (see bonding) an atom can form. Hydrogen (H) always has valence 1, so other elements' valences equal the number of hydrogen atoms they combine with. Thus, oxygen (O) has valence 2, as in water (HO); nitrogen (N) has valence 3, as in ammonia (NH); and chlorine (Cl) has valence 1, as in hydrochloric acid (HCl). The valence depends on the number of unpaired electrons in the outermost (and, in transition elements, the next) shell of the atom's structure. The sharing of the unpaired (valence) electrons in a bond mimics the stable configuration of the noble gases, whose outer shells are full. Elements that can achieve stable configurations by various combinations have more than one valence.

This entry comes from Encyclopædia Britannica Concise.
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