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.