Linus Pauling: Now we may discuss the structure of all of the elements, all of the elements that
make up the world as we know it in relation to the periodic system of the elements.
Let us represent the various elements by showing how their electrons occupy orbitals,
and we can plot energy vertically.
We start out with the 1s orbital. Hydrogen can have one electron in the 1s orbital
- I represent it by putting in an arrow pointing upward. Helium can have a second
electron – two electrons in the 1s orbital. I show them the arrow pointing, pointing
down. By Power’s Principal, that is all the orbitals the electrons that can be placed
in the 1s orbital.
The next orbital is 2s. When we come to lithium, the lithium atom with three electrons
can have two in the 1s orbital, giving a completed helium shell, and then one in 2s.
Beryllium, with atomic number four, and four electrons, can be represented by having
two electrons in the 2s orbital, its electron configuration is 1s2, 2s2. Then we
have the three 2p orbitals. They can be occupied successively by electrons in boron,
carbon, nitrogen, oxygen, fluorine, neon. At neon, this shell too, is completed –
the neon shell.
Now we come to the third shell in the periodic table. The 3s orbital, two 3p orbitals
and…that’s all that I want now, two 3p orbitals. The succession of elements here,
sodium, magnesium, scandium, let me see, silicon, phosphorous, sulfur, chlorine, argon.
This succession of eight elements brings us up to the noble gas argon, and we can
say that this third shell is the argon shell.
Next, we come to the fourth shell, with atomic number nineteen, potassium. Here we
have two electrons, ten electrons, eighteen electrons. These numbers, two, ten, and
eighteen, are characteristic of the first three noble gases. With potassium, we have
the 4s electrons. With calcium, then, a pair of 4s electrons. Then, the 4p orbitals
and the 4d orbitals. One, two, three, four, five of the 3d orbitals. This shell
is called the krypton shell, a shell of eighteen electrons occupying nine orbitals...from
here, whereas here in the shorter shelves we have only four. These can be occupied
by eighteen elements: potassium, calcium, scandium...I think that I said scandium
down here, of course, here, I should have said sodium, magnesium, aluminum, silicon,
and so on, here we have the first long period of eighteen elements: potassium, calcium,
scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc,
gallium, germanium, arsenic, selenium, bromine, and krypton. This is, this ends up
with krypton and we call it the krypton shell of eighteen elements.
Here, I should mention that the usage of this term “shell” varies somewhat. It has
been customary to refer to the k-shell, the l-shell, and the m-shell and so on. Well,
the m-shell, in the old designation, is the shell that contains all of the orbitals
with the same total quantum number; 3s, 3p and 3d. But, so far as we are concerned,
in chemistry, it is more important to lump together the orbitals with roughly the
same energy, and here, these are the 4f orbitals, the three 4p orbitals, and the five
3d orbitals.
The next shell, the next shell, is the second, involves the second long period, again
of nine orbitals, 5s, 5p, 4d, and eighteen elements ending up with xenon. Xenon, Xe,
and this shell of eighteen electrons, nine orbitals, we can call the xenon shell.
The next shell, 6s, then, the 6p orbitals, five 5d orbitals, and seven 4f orbitals
comprises the radon shell. This is a very long shell. Sixteen orbitals altogether,
thirty-two elements. It brings us up to atomic number, well, let’s see, helium has
atomic number two, neon, ten, argon, eighteen, krypton, thirty-six, radon, fifty-four.
Thirty...no, xenon, 54, radon, eighty-six. Following this comes the second...very
long shell 7s, 7p, 6d, 5f, another thirty-two elements that would bring us up to element
one hundred eighteen, that we may call eka-radon.
