Linus Pauling: During the preceding two lectures, we have discussed the electronic structure of
atoms and molecules and some aspects of valence; ionic valence involving a transfer
of electrons from one atom to another, covalence, the sharing of a pair of electrons
between two atoms, normal covalence if the atoms are atoms of the same element...Covalence,
with some ionic character, if the atoms are atoms of different elements with different
electronegativities. This, the idea, this idea, the idea of covalent bonds with partial
ionic character, is one illustration of the progress in chemistry that has been made
because of the development of the theory of resonance.
Now, we want to take up some other aspects of the general subject of valence and molecular
structure, aspects such as ligancy or coordination number. Metallic valence, the
nature of the forces that hold atoms of copper together in the metal copper. Oxidation
numbers, a part, an idea in the field of valence theory that is useful in balancing
oxidation-reduction equations. The hydrogen bond, well, this is getting us into the
question of the weak forces that operate, the relatively weak forces that operate
between molecules.
Now, let me mention two or three aspects of covalence. I have here a model of sulfur,
the sulfur molecule, S8, as it appears in ordinary rhombic sulfur. It fits in very well with the general
theory of valence that we have discussed. I can’t draw the whole ring, we have each
sulfur atom forming two bonds and having two electron pairs. These are the four orbitals
that correspond to the argon shell. Sulfur has completed its argon structure in forming
a molecule of this sort.
Hydrogen chloride is a molecule that I mentioned, HCl, in which we have a covalent
bond with about twenty-percent partial ionic character. I want to mention that we
must not confuse partial ionic character in the hydrogen chloride gas molecule with
the ionization of hydrogen chloride, hydrochloric acid, in aqueous solution. These
are two different matters. In aqueous solution, hydrogen chloride, hydrochloric acid,
is a strong electrolyte, completely ionized. It forms hydrogen ions or, perhaps we
should say, hydronium ions in which a hydrogen is attached, an extra hydrogen ion
is attached to a water molecule, oxygen has completed its octet, just as in water
itself, but in this case it has the neon structure, just as in water itself, in this
case it has three unshared – three shared pairs and one unshared pair in the neon
valence shell.
A related question is the question of the use of orbitals that are not involved in
the valence shell of the nearest noble gas, the noble gas with somewhat larger atomic
number. Let me use silicic acid and the, the related acids, phosphoric acid, sulfuric
acid, perchloric acid as an example. Silicic acid is SiOH4 and we can draw a structure for it in this way, as G. N. Lewis first did, in which
each of the oxygen atoms has achieved the helium, the neon structure. In the same
way for phosphoric acid, we can show P, O, O, OH, OH. For sulfuric acid, S, O, O,
H, H, O, and perchloric acid, Cl, OH, O, O, O. In each of these structures, the central
atom is shown as having achieved the argon configuration of electrons. But, the interatomic
distances observed for these acids are such as to indicate that there is a considerable
amount of double-bond character in the silicon-oxygen bonds, the phosphorous-oxygen
bonds, the sulfur-oxygen bonds, and the chlorine-oxygen bonds. This double-bond character
could be achieved by making use of orbitals that are in the next shell beyond the
argon shell.
I may make mention of the acid strengths of these acids. There is a simple consideration
that leads to an understanding of the observed acid strengths. Silicic acid, H4SiO4, is a very weak acid, only a little stronger an acid than water itself. Now here
we have an OH. If the hydrogen ion ionizes away, this ion is left with a negative
charge and it is the attraction of this oxygen for the hydrogen ion that makes the
acid a very weak acid. If, however, the hydrogen ion ionizes away from phosphoric
acid, we see that there are two oxygens left that are equivalent to one another so
that we might say that there is a charge of minus one-half, the total charge minus
one of H2PO4 is divided between these two oxygens. Neither one of them attracts the proton, as
it approaches, so strongly as this one oxygen with charge minus-one attracts the proton
in silicic acid, and in fact, phosphoric acid is much stronger than silicic acid –
it is still classed as a weak acid.
When sulfuric acid ionized, we can say that there is a charge of minus-one-third on
each of these three oxygens of the HSO4 ion, HSO4- ion, and so there is a still weaker attraction for the proton as it approaches, as
it is attracted to all three, no one of them so strongly as in the H2PO4- ion. Sulfuric acid is classed as a strong acid, and of course with perchloric acid,
when the proton ionizes away, we have a charge of minus one-quarter on each of the
oxygens. Perchloric acid is a very strong acid.
