Linus Pauling: We shall start the discussion of the electronic structure of molecules, the electronic
interpretation of valence, by discussing the electronic structure of atoms. We know
a great deal about the electronic structure of atoms nowadays, and it is the physicists
who have determined the information for us. The sort of experimental material that
they have used in finding out about electronic structure of atoms is mainly the spectra,
the light emitted by atoms, by substances when they are strongly heated or subjected
to the action of an electric spark or an electric arc.
The first precise description of an atom, not exactly right, was given by Niels Bohr
over forty years ago now. Bohr described the hydrogen atom in its normal state in
the following way: He said there is a small, heavy nucleus, the proton, and an electron
which moves about it in a circular orbit. The radius of the circular orbit was given
by his calculations as 0.530 angstrom and the speed with which it moves in its orbit
as 2.18 X 10 to the 8th centimeters. That is a little less, two-thirds of one percent
of the speed of light.
Now, the modern picture of the hydrogen atom in its normal state is somewhat different.
We describe the hydrogen atom now as consisting of the same central nucleus, the proton,
and the electron, which instead of moving circularly, moves in and out. It is known
that the electron does not have angular momentum in its orbit; it is not moving sideways,
but only in and out. The average speed, the root-mean square speed with which the
electron moves is just the speed that was assigned to the electron by Bohr. And the
average distance of the electron from the nucleus is the same as the radius assigned
forty years ago by Bohr, to the circular orbit of the Bohr atom.
In addition to this orbit for the normal state of the hydrogen atom, there can be
excited states. Bohr talked about a larger circular orbit as representing the first
excited state, or an elliptical orbit. According to quantum mechanics, the next most
stable orbit for a hydrogen electron and a hydrogen atom, is another one in which
the electron moves in and out about the nucleus. The third most stable one is one
in which the electron moves in essentially an elliptical orbit such that there is
some sideways motion too, some angular momentum.
The normal state is represented by the symbol 1s. We talk about the 1s orbital.
Then the next state by the symbol 2s and then the symbol 2p, and there are three kinds
of orbits with the symbol 2p. We may speak of them, think of them, as having the
motion in the plain of the blackboard or in the plain at right angles to the plain
of the blackboard this way, or in the third plain, at right angles to both of the
other two plains. There is the most stable orbital, the 1s orbital, the next most
stable, 2s, and then three 2p orbitals. The electron has a spin, as discovered in
1925 by Uhlenbeck and Goudsmit, and this spin of the electron can orient itself in
two ways. Either, let’s say, with the Earth’s magnetic field or against, opposed to
the Earth’s magnetic field either parallel, or anti-parallel.
The Pauli Exclusion Principal, discovered by Pauli in, I think, 1925 or 1926...Pauli
Exclusion Principal states that no two electrons in the universe can be in exactly
the same state. If the two electrons are moving around the same nucleus, for example,
if we consider a helium atom with a nuclear charge of plus two, there can be one electron
in a 1s orbital, and then a second in a 1s orbital, the same orbital, provided that
its spin is opposed, so that one of them has positive spin and one has negative spin.
There is a permanent magnetic moment associated with the spin of the electron. We
can think of this as corresponding to a small magnet. The spin must be oriented with
the North Pole up in the one case for the one electron, and the North Pole down for
the other so that the two little magnets neutralize each other’s magnetic fields and
helium turns out not to have a magnetic moment.
The third electron in an atom such as lithium will have to occupy another orbital.
The 1s orbital is completely occupied when it has two electrons in it as at helium.
This is indicated by putting the superscript 2 on 1s. 1s squared is the symbol for
the electron configuration of the helium atom.