Linus Pauling: Here, I have the next alkali metal ion, the cation potassium, charge of nineteen, one electron missing from it, the outermost
electron beyond the argon shell is easily removed, leaves it with eighteen electrons and a positive charge. The electronic
structure is almost identical with that of the chloride ion. The two innermost electrons have shrunk in a bit, the eight
electrons, the eight electrons of the neon shell have shrunk a bit, and the eight electrons of the argon shell have shrunk
in a bit. It is evident that we would expect the radius, the effective radius, the effective size of the potassium ion, to
be somewhat less than that of the chloride ion. Their dimensions are, potassium, I think, 1.33 angstrom, chlorine, 1.81 angstrom.
Now we move on to bromide ion, atomic number 35, 36 electrons, two in the helium shell, eight in the neon shell, eight in
the argon shell, and eighteen in the krypton shell. Rubidium is the iso-electronic ion, 37 is its atomic number, it has lost
one electron, the thirty-six electrons are arranged in the same shells; two in helium, eight in neon, eight in argon, eighteen
in the krypton shell, and the ion has shrunk in size compared with the bromide ion.
The element iodine, with atomic number fifty-three, is just one short of xenon. It can pick up one electron, getting fifty-four
electrons, and its electronic structure is as shown here: two k electrons very close in to the nucleus, then eight electrons
in the argo-, in the neon shell, eight in the argon shell, eighteen in the krypton shell, and eighteen in the xenon shell.
Iodine, the iodide ion, is somewhat larger that the bromide ion, which is somewhat larger than the chloride ion and so on.
Cesium loses one of its fifty-five electrons easily to assume the electronic structure of xenon. It has two k electrons,
eight in the neon shell, eight in the argon shell, eight in the krypton shell, and eight in the xenon shell. The sizes of
ions, the sizes of these alkali ions and halogenide ions, and also the sizes of the other ions, beryllium, double-plus magnesium,
double-plus scandium, or calcium, double-plus strontium, double-plus barium, double-plus and so on, are of much value in the
discussion of the properties of substances.
Sodium chloride, when heated very strongly, forms diatomic gas molecules, NaCl. It also forms some more complicated molecules,
Na2Cl2 and so on. These easily can condense to form sodium chloride crystals. The structure of the sodium chloride crystal is
indicated by this model. This model is about on the scale of one inch to one angstrom, linear magnification two hundred and
fifty million fold, so that it isn’t a very good representation of the big crystal of sodium chloride that we have, that we
saw earlier in this lecture. If we wanted to make this model represent the structure of sodium, of the crystal of sodium
chloride, this one, we would have to continue it on until it was about three times the diameter of the Earth. That is, its
volume would be about twenty-seven times that of the Earth if the atoms were this big.
Well, here we have the chloride ion and the sodium ion on the scale corresponding to their relative electron distributions;
1.81 angstroms for the radius of chlorine and about 0.95 angstrom for the radius of sodium. They are arranged in this way
in the sodium chloride crystal. Each sodium ion is surrounded by an octahedron of six chloride ions, and each chloride ion
is surrounded by an octahedron, four here, one behind and one in front, of six sodium ions. The ligancy, or coordination
number, of the sodium ion is six in this structure.
It is easy to understand why sodium chloride has a cubic cleavage. You see, here in this front face, there are nine, ten,
eleven, thirteen of these chloride ions and twelve sodium ions, but if it were a bigger face, there would be exactly the same
number of sodium ions and chloride ions so that this layer of the crystal would be electrically neutral. We could expect
then, that it might be possible to split off the electrically neutral layer and in this way achieve the cubic cleavage of
the crystal. On the other hand, if we cut along this diagonal, perpendicular to the body diagonal of the cube, we have first
a layer of chloride ions with negative charge, then a layer of sodium ions with positive charge, then a layer of negative
ions with chloride, with, of chloride ions with negative charge, which would be hard to separate the successive charged layers
from one another.