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.
