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.