Linus Pauling: There are many other properties of substances that can be discussed in a straightforward way on the basis of the electronegativities
of the atoms and the partial ionic character of the bonds. The relation between the energy that is liberated and the difference,
or when a bond between two atoms is formed, and the difference in electronegativity of the atoms is the following: The extra
energy, resonance energy, due to the partial ionic character of a covalent bond is approximately equal to twenty-five kilocalories
per mol, twenty-five kilocalories per mol times the square of the difference in the electronegativities of the two atoms.
I can write Xa minus Xb where X represents the electronegativity. Xa is the electronegativity of atom A while Xb is the electronegativity
of atom B. The difference, squared, multiplied by twenty-five kilocalories per mol, gives the heat of formation of the bond
and multiplied by twenty-five percent, gives the partial ionic character of the bond.
This explanation of the heat of reactions, all sorts of reactions, applies to substances containing single bonds. One must
be careful not to try to apply it to substances involving double bonds or triple bonds because the double bond and the triple
bond have characteristic bond energies associated with them.
We have talked about ionic bonds and about covalent bonds and about bonds that represent resonance between ionic bonds and
covalent bonds, bonds with partial ionic character. This doesn’t exhaust modern valence theory. There are still such questions
of what is the nature of the bond that holds together the copper atoms in a copper crystal, where each atom is similarly situated
with respect to its twelve neighbors? Or what about the hydrogen bond? What about oxidation numbers of atoms? Well, it
is questions of this sort that we shall come back to in our next lecture.