Ionization energies describe how unwilling an atom is to lose an
electron, and electron affinities measure how eager a neutral atom
is to add one more electron. Both of these quantities describe the
attraction of an atom for an electron under special conditions.
Electronegativity is a more general measure of the attraction of
an atom for an electron in bond-forming situations.
Electronegativities could be calculated from ionization energies
and electron affinities, but in practice they are obtained by measuring
the strengths of different types of bonds in molecules.
Robert Mulliken and Linus Pauling independently devised the electronegativity
concept. In the Pauling electronegativity scale, the electron-losing
lithium atom is assigned an electronegativity (or EN) of 1.0, the
electron-grabbing fluorine is given an EN of 4.0, and the other
atoms are assigned intermediate values, as shown on the
next page. In a bond between two atoms, the electrons will be
attracted more strongly to the atom with the greater electronegativity.