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.