Linus Pauling: We have then, a complete explanation of the periodic system of the elements. The
helium shell, one orbital, two elements. Outside of that is the neon shell 3s, 2s
orbital, and three 2p orbitals, eight more electrons bringing us up to atomic number
ten. Then the argon shell of eight electrons, the krypton shell of eighteen electrons,
the xenon shell of eighteen, the radon shell of thirty-two, and, going on to the next
shell with thirty-two elements, it would end up at a hundred eighteen.
The elements from titanium, from scandium on to zinc, we may call the transition elements
of the iron group. They correspond to putting ten electrons into the five 3d orbitals.
Similarly, the elements from deuterium on to cadmium we can call the transition elements
of this group. Following lanthanum, there come fourteen elements, the lanthanons,
from cerium to lutetium that correspond to the introduction of fourteen electrons,
one at a time, into the seven 4f orbitals.
A great deal is known about the distribution of electrons in the various atoms and
ions. This information has been obtained in part by experimental methods an in part
by theory, by quantum mechanical theory. The results of x-ray experiments and electron
diffraction experiments have shown that the electron distributions agree well with
those that have been calculated by theoretical physicists using the Schrödinger wave
equation, the fundamental equation in quantum mechanics.
I have some drawings that I made, in fact, twenty-four years ago, when I was going
to give a lecture in Santa Barbara, some drawings that show how the electrons are
distributed in the various ions. I, I don’t remember just why I was so interested
in ions at that time, but I made the drawings for, for example, the bromide ion, a
bromine atom that has picked up an extra electron, and for the rubidium ion, a rubidium
atom that has lost one electron. These drawings are a good representation, I believe,
pretty accurate rough representations, of the electron distributions in the various
ions.
Here, I have the drawing of lithium. The nucleus at the center, the lithium ion,
Li+, the nucleus at the center, and then two electrons constituting a completed helium
shell that moves in and out and have a distribution in space that is indicated by
this drawing.
As we go on across the periodic table, from lithium towards the end of the first short
group, we come to the fluorine atom which, being just short of neon, can pick up one
additional electron. The electron, electronic structure of the fluoride ion is shown
here. The nucleus, two electrons close in to the nucleus that constitute the helium
shell, shrunk in because the nuclear charge is large, much larger than for lithium,
and then eight electrons in this fluoride ion, two of which are moving in and out
radially, the other six in somewhat elliptical orbits with the electron distribution
shown here. You see that the fluoride ion is several times larger, roughly three
times the diameter of the lithium ion.
Then here is the sodium ion. Sodium ion, sodium has eleven electrons. One of them
has been lost to form the sodium ion, Na+, leaving ten electrons just as in the fluoride ion. These ten electrons are arranged,
are distributed as shown here. The two in the helium shell are close in, the eight
that constitute the neon shell are also shrunk in somewhat from the dimensions in
the fluoride ion. This is the effect of the increase by about twenty percent in effective
nuclear charge on going from fluorine to sodium. The scale is shown here, one angstrom.
The sodium ion is about one angstrom in radius. The conventional crystal radius for
sodium ion is 0.95 angstrom.
From sodium we continue across the first short period of the Periodic Table to chlorine,
which is just one short of argon. The chlorine atom picks up an electron easily to
form the chloride ion and the structure of the chloride ion is as indicated here.
Two electrons in the k shell, the helium shell, then eight in the neon shell, and
eight more in the argon shell, giving the ion with one negative charge.
