There are a number of ways to produce a set of numbers which represent electronegativity scales. The Pauling scale is perhaps the most famous. Linus Pauling noticed that the bond energy E(AB) in a molecule AB is always greater than the mean of the bond energies E(AA) + E(BB) in the homonuclear species AA and BB. His argument was that in an "ideal" covalent bond E(AB) should equal this mean, and that the "excess" bond energy is caused by electrostatic attraction between the partially charged atoms in the heternuclear species AB. In effect, he was saying that the excess bond energy arises from an ionic contribution to the bond. He managed to treat this ionic contribution by the equation
E(AB) = [E(AA).E(BB)]1/2 + 96.48(ΧA - ΧB)2
in which E(AB) is expressed in kJ mol-1 (1 electron volt, 1eV, = 96.48 kJ mol-1) and ΧA - ΧB represents the difference in "electronegativity" between the two elements, whose individual electronegativities are given the symbols ΧA and Χ. Using this equation, Pauling found that the largest electronegativity difference was between Cs and F. Pauling set F arbitrarily at 4.0 (today, the value for F is set to 3.98) and this gives a scale in which the values for all other elements are less than 4 but still with a positive number.