Linus Pauling: Well now, the way that chemists have attacked the problem of the relation between
the properties of substances and their structure, well, there are various ways. Perhaps
the most important way goes back about a hundred years now. It was just a hundred
years ago that Frankland, Kekule and Couper, and other people making their contributions,
originated the idea of the chemical bond and valence. It was known at that time,
a hundred years ago, that substances such as salt, have formulas such as NaCl, hydrogen
fluoride is HF, water H2O; ammonia, NH3; methane, CH4.
It was suggested that there are bonds that hold the atoms together; Na to Cl, H to
F, water, H to O and another H-O bond, with ammonia, N-H, N-H, N-H, and with methane,
C-H, C-H, H, H. Hydrogen and fluorine, sodium and chlorine, are said to be univalent,
to have valence one. Water, oxygen is bivalent; it can form two chemical bonds.
Nitrogen is tervalent, carbon, is quadrivalent. I have here some models illustrating
this; standard ball and stick models. H2O, NH3, CH4.
Many, much of the development of chemistry during the last hundred years has been
the result of the development of chemical structure theory. Chemists have learned
how to arrange the valence bonds in a drawing, a structural formula of a substance.
But it has been found that the problem is really not a completely simple one. The
idea of valence in the old-fashioned, rather vague form, has been found to be unsatisfactory
and during recent decades, especially in the period beginning about 1916, this concept
of valence has been replaced by several more precise concepts. The concept of ionic
valence, the concept of covalence, of metallic valence, of some others too that have
less general significance. In order to understand these more precise concepts of
valence, we have to know something about the electronic structure of atoms.
Let us start out with a discussion of ionic valence. Here we have the periodic system
of the elements. The elements are arranged in order of their atomic numbers. Hydrogen,
the simplest atom, consists of a nucleus with electric charge plus-one in electronic
units, and a single electron outside of the nucleus. Helium has two electrons outside
of the nucleus with charge plus-two. Lithium has three electrons outside of a nucleus
with charge plus-three and so on. Neon, here, has ten electrons surrounding a nucleus
with charge plus-ten.
Now, the electronic structures characteristic of two electrons as in helium, ten electrons
as in neon, are especially stable. This is the reason that helium and neon do not
form chemical compounds of the ordinary sort in the way that hydrogen, lithium and
other elements form chemical compounds. The third electron on lithium is held only
loosely by the atom. It is easy to pull that electron away from the lithium atom.
Moreover, fluorine, with nine electrons, has a considerable affinity for an additional
electron. It can pick up an electron. The result of this is that if lithium metal
and fluorine gas come together, there is a vigorous chemical reaction that leads to
the formation of the salt, lithium fluoride.
The structure of lithium fluoride is the following one: lithium has lost one electron,
become the lithium ion with the same number of electrons as helium. Fluorine has
gained one electron, and become the fluoride ion with the same number of electrons
as neon. Then, there is the electrostatic attraction between these ions of opposite
electric charge – the same sort of attraction that operates between two pith balls,
one of which has a positive charge and one a negative charge – between any two objects
that have electric charges of opposite sign. It is this strong electric attraction
between the ions, the lithium ion and the fluoride ion, that holds these ions together
and the lithium fluoride gas molecule, which you obtain at high temperatures when
lithium fluoride is strongly heated, or in the lithium fluoride crystal.