electronegativity [key], in chemistry, tendency for an atom to attract a pair of electrons that it shares with another atom (see chemical bond). For example, the molecule hydrogen chloride, HCl, consists of a hydrogen atom, H, and a chlorine atom, Cl, sharing a pair of electrons. If the pair of electrons are not shared equally, i.e., if they spend more time with one atom than with the other, the favored atom is said to be more electronegative. In the case of HCl, measurements indicate that the molecule has a dipole moment, that is, the chlorine end is relatively negative and the hydrogen end is relatively positive. This means that the electron pair spends more time with the chlorine atom than with the hydrogen atom and thus chlorine is more electronegative than hydrogen. Nonmetals have much higher electronegativities than metals; of the nonmetals, fluorine is the most electronegative, followed by oxygen, nitrogen, and chlorine. The larger the difference in electronegativity between two atoms, the more polar the bond between them. In the extreme case of a bond between a metal and a nonmetal, a complete transfer of electrons takes place.
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