Linus Pauling: There is another aspect of the theory of valence that I should mention now, and this
is one that has been understood, at any rate understood to the extent that we shall
discuss now, only during the last few years. This is the matter of metallic valence.
What is it? What is the nature of the forces that hold the copper atoms together
in the metal copper, in the crystal of copper? Well, let us consider copper. I think
that I would prefer to consider aluminum. Let us consider the metal aluminum. It
has the same structure as copper – cubic closest packing as shown here. Each atom
of aluminum has, each atom of aluminum here is surrounded by twelve neighbors that
are equally distant from it; six in this layer, three in the layer behind, three in
the layer in front.
Now, aluminum has atomic number thirteen. It has three electrons outside of the neon
shell, plenty of orbitals, four orbitals in the argon shell, so that we would expect
it to form three covalent bonds using its three electrons. In order that it be bonded
equally to twelve neighbors, we may describe the three bonds as resonating among the
twelve structures and holding the aluminum atoms together.
The same sort of resonance of covalent bonds among a large number of alternative positions
occurs in other elements. For example, in the metals potassium, calcium, scandium,
titanium, vanadium, chromium, the properties of these metals correspond nicely to
the idea that we have one, two, three, four, five, six electrons, six bonds formed
by each atom and resonating around the positions connecting the atom with the neighboring
atoms. The malleability and ductility can be understood in terms of this resonance,
also the property of electric conduction, the ability of the metal to conduct the
electric current. I may mention that there are two kinds, or two common kinds, of
closest packing of spheres that are represented by the metals. In, this is cubic
closest packing, analogous to this structure, along a three-fold axis of the structure,
there is repetition after three layers. There are two alternative ways of placing
any layer above the layer beneath it: this way, or this way. In cubic closest packing,
these layers, these ways, repeat so as to give repetition after three layers, in hexagonal
closest packing, there is repetition after two layers. This gives a hexagonal crystal.
Magnesium and many other metals have this structure. Aluminum, copper, silver, gold,
many other metals have this structure, cubic closest packing.
I think that it is interesting that in 1890, many years before x-ray diffraction was
discovered, the English amateur scientist, William Barlow, had assigned this hexagonal,
closest packed structure to magnesium on the basis of the knowledge that magnesium
crystallizes at hexagonal crystals, with the right, with a certain dis-, ratio, this
distance to this in the crystal, and had assigned cubic closest packing to aluminum,
copper, silver, gold, and other metals. In fact, he had assigned the sodium chloride
structure to sodium chloride and other alkali halogenides, the cesium chloride structure
to cesium chloride, the sphalerite structure to the cubic form of zinc sulfide, the
wortzite structure to the hexagonal form of zinc sulfide, the fluorite structure to
fluorite, CaF2, and he was right on all of these assignments. He wasn’t, wasn’t sure, of course,
that he was right, but it has turned out that his ideas about closest packing of spheres
and so on were right and permitted him to find the correct structures.