Linus Pauling: This simple consideration applies also to acids of other sorts: boric acid, H3BO3, is a very weak acid because it has only OH groups attached to the central atom. A standard example of a molecule that exceeds
the octet is phosphorous pentachloride, PCl5. The conventional formula, structural formula for this molecule, is the one shown here in which each chlorine atom has achieved
the argon structure. It is known that the molecule has the configuration of a trigonal bipyramid: two chlorines above, three
around the equator. The structure as indicated here causes phosphorous to form five bonds and this would require that phosphorous
make use of one orbital beyond the orbitals in the argon shell, four orbitals in the argon shell.
This, however, is not the only structure that we may write for phosphorous pentachloride. We may write a structure such as
this one; three chlorines bonded around the equator, the atoms of course must be in the same position for electronic resonance,
resonance of the bonds, and a chloride ion in this position, a positive charge on the phosphorous atom, this P+, positively charged phosphorous, is now forming four covalent bonds using only the four orbitals of the argon shell and one
ionic bond. There are five structures of this sort in which the five chlorine atoms are successively given a negatively charge,
given a negative charge, and a resonance structure involving all of these introduces just about the right amount of partial
ionic character to the phosphorous-chlorine bonds, that would be sixteen percent of partial ionic character to the phosphorous-chlorine
bonds, corresponding pretty well to the difference in electronegativity of phosphorous and chlorine in the electronegativity
scale.
There is another aspect of valence theory that I should like to discuss now. This is ligancy, or coordination, the coordination
of several atoms or groups of atoms around the central atom. For example, here in the sodium chloride crystal, we have sodium
ion that, where is it, sodium ion surrounded by six chloride ions in an octahedral arrangement, chloride ions surrounded by
six sodium ions. It is customary to refer to the cation usually as the coordinating ion and to say that sodium has ligancy
six in the sodium chloride crystal. Its ionic valence is one. We can say that it forms six, one-sixth ionic bonds with the
six surrounding chloride ions. In beryllium, in the hydrated beryllium ion, BeH2O four times, there are four bonds formed between beryllium and the surrounding water molecules. In the hydrated magnesium
ion, Be, or MgH2O six times, there are six water molecules around the magnesium ion located at the corners of an octahedron. In BeH2O four times, they are at the corners of a tetrahedron. Now, the bonds between the beryllium ion, or the water, or the magnesium
ion, or aluminum ion in AlH2O six times, and the oxygen molecule, involves the electrons pairs, an unshared electron pair of the water molecule. These
bonds are not, however, normal covalent bonds. They are covalent bonds with partial ionic character.
Here we have beryllium, here magnesium and aluminum in the electronegativity scale, and oxygen over at 3.5, nitrogen at 3.0.
With a metal ion and oxygen or nitrogen in the water molecule or the ammonia molecule, the bonds have only one-third to one-half
covalent character, two-thirds to one-half ionic character so that there is not a great amount of electric charge transferred
from the oxygen atom of water or the nitrogen atom of ammonia to the central atom. In fact, the amount of partial covalent
character of these bonds is just about enough to neutralize the charge on the central atom, to leave it electrically neutral.
We may say that there is a sort of electro-neutrality principle operating here, that atoms strive to have zero charge rather
than the charge of plus two for beryllium and magnesium, plus three for aluminum.
