What is Electronegativity?

The electronegativity of an element measures how strongly its atoms, in a molecule, attract the electrons of the bonds they share. It is a number without unit; on the usual scale it runs from about 0.8 to about 4.

The electronegativity $\chi$ of an element measures the tendency of its atoms, in a molecule, to attract the electrons of the bonds they form. Two scales are in use.

• On the Pauling scale, differences are defined from bond energies $D$:

  $$|\chi_A - \chi_B| = \sqrt{\frac{\Delta E}{1\,\mathrm{eV}}}, \qquad \Delta E = D(\text{A--B}) - \tfrac12\big[D(\text{A--A}) + D(\text{B--B})\big],$$

  and the scale is anchored by $\chi(\ce{F}) = 3.98$.

• On the Mulliken scale, $\chi_M = \tfrac12\,(E_{i1} + E_{ea})$, in electronvolts.