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.
