Linus Pauling: There is another question that we can answer with use of the theory of resonance
and in answering it, we achieve a great simplification of chemistry, coordination
of a great number of facts of inorganic chemistry.
I may use hydrogen chloride as an example in discussing this question. What is the
structure of hydrogen chloride, HCl? Well, I can write its Lewis structure in this
way. There’s a bond, a covalent bond, between hydrogen and chlorine. Hydrogen has
achieved the helium structure, chlorine the argon structure. But of course, if I
were giving the talk on ionic valence, I might say I’ll write H+ Cl-. And thirty years ago, there was much argument as to which of these structure, the
ionic structure, or the covalent structure was the correct one.
Well, we know the answer now. The theory of resonance [unintelligible] can be written
for hydrogen chloride. A normal covalent structure, similar to the structure in the
hydrogen molecule and the structure in the chlorine molecule, intermediate between
these structures, and an ionic structure, with chlorine negative and hydrogen positive.
Well, now we may ask, how, to what extent do these two structures contribute? And
there are several ways of getting an answer to this. In the hydrogen chloride molecule,
the distance between the nuclei is 1.27 angstrom. The electric dipole moment of HCl
is known. It corresponds to a charge plus 0.2 on hydrogen and minus 0.2 on chlorine.
We can say, then, that there is about twenty percent ionic character and eighty percent
covalent character to the hydrogen chloride molecule.
In fact, in fact, it is possible to correlate the amount of ionic character of molecules
containing single bonds, with a scale, an electronegativity scale, starting with fluorine,
4.0, oxygen, 3.5, nitrogen, 3.0, carbon, 2.5, hydrogen, 2.1, boron, 2.0, beryllium,
1.5, lithium, 1.0, chlorine, 3.0, sulfur, 2.5, bromine, 2.8, iodine, 2.4. And, the
amount of partial ionic character depends upon how far apart the elements are in this
electronegativity scale.
You see, this is a sort of skewed Periodic Table. Instead, the halogens, instead
of following the line directly below the one, are skewed over in this way. The most
electronegative element is fluorine, the next most electronegative element oxygen,
and so on across. Now, elements, differ in electronegativity by about one unit.
Chlorine and hydrogen, 0.9. Then, there is about twenty percent partial ionic character.
If they differ in electronegativity by about two units, the partial ionic character
is more than fifty percent, some sixty or seventy percent.
This electronegativity scale was set up from the consideration that whenever there
is resonance between two structures, a substance is stabilized. Benzene is much more
stable than an ordinary unsaturated compound involving a double bond. It is the resonance
energy between the two Kekule structures that provides the extra stabilization.
Hydrogen chloride is more stable than it would be if it had a normal covalent structure.
H2 plus Cl2 forms 2HCl with the liberation of 2 x 22 kilocalories per mol of energy. The bond,
HCl bond, this twenty-two kilocalories per mol, per mol more stable than the average
of the H-H bond and the Cl-Cl bond. Hydrogen and fluorine, H2 plus F2 form 2HF with 2 x 64 kilocalories per mol. Hydrogen and bromine form 2HBr with 2
x 12. Hydrogen and iodine form 2HI with 2 x 2 kilocalories per mol. Very nearly
zero.
Now, iodine, 2.4, and hydrogen, 2.1, have nearly the same electronegativity. Consequently,
the HI bond is almost a normal covalent bond, very little partial ionic character,
and correspondingly, the bond is hardly anymore stable than the average of the bond
for a hydrogen molecule and an iodine molecule.
Bromine is somewhat more electronegative than hydrogen, 2.8 against 2.1, and the bond
is twelve kilocalories per mol more stable. Chlorine is still more electronegative,
difference of 0.9, the bond is twenty-two kilocalories per mol more stable. For fluorine,
sixty-four kilocalories per mol more stable
As a rough approximation, we can say that the partial ionic character of a bond is
equal to the amount of extra stability of the bond, as given by the heat of formation
of the substance in kilocalories per mol. About two percent partial ionic character
in HI, twelve percent in HBr, twenty-two percent in HCl, sixty-four percent in HF.
Now, knowing the electronegativities of elements, we can make predictions about the
heats of formation of all substances that involve single bonds. I can take, for example,
graphite and hydrogen to form methane. What would be the heat of the reaction? Graphite
plus hydrogen to form methane. Carbon, 2.5, hydrogen, 2.1, the bond corresponds to
about four tenths difference. I know by comparison with HI and HBr, there will be
somewhere around five kilocalories per mol extra stability of the hydrogen-carbon
bond, and for a methane molecule with four of these bonds then, about twenty kilocalories
per mol as the heat of formation of the molecule.
