The concept of electronegativity was first introduced by Linus Pauling in the 1930s as a means of describing bond energies. Bond energies of polar bonds (formed by atoms with different electronegativities) are larger than the average of the bond energies of the two homonuclear species. For example, HC1 has a bond energy of 428 kJ/mol, compared
to a calculated value of 336 kJ/mol, the average of the bond energies of H2 (432 kJ/mol) and C12 (240 kJ/mol). From data like these, Pauling calculated electronegativity values that could be used to predict other bond energies. More recent values have come from other molecular properties and from atomic properties, such as ionization energy and electron affinity. Regardless of the method of calculation, the scale used is usually adjusted to give values near those of Pauling to allow better comparison. Table 3-4 summarizes approaches used for determining different scales.