Just as the earth spins on its axis as it orbits
the sun, so the electron spins on its axis as it
orbits the nucleus.
This was not the end of the
matter. Even more accurate experiments revealed additional
spectral lines. This was called the 'Anomalous Zeeman Effect'
and it drove the Austrian (but later Swiss) theoretician
Wolfgang Pauli crazy! Pauli realized that an, as yet, observed
property of the electron needed to be accounted for. He
wondered if the electron could spin around its own axis
as it orbited the nucleus – just as the spinning earth
revolves around the sun. In the end, he found the idea unsatisfactory
and never published it. A pair of Dutch physicists in their
mid-twenties, George Uhlenbeck and Sam Goudsmit beat Pauli
to it and published the idea of electron spin. Pauli's lack
of conviction cost him the claim to the idea of electron
The theory predicted that electrons can be described
as either 'spin-up' or 'spin-down'. Nothing is simple
in the quantum world though. The electron must perform
two revolutions in order to return to the same point!
Pauli was not discouraged and went on in 1925 to explain
why everyday objects are solid, i.e.: why all the electrons
do not fall into the nucleus, which would make all atoms
unstable. This was a big problem with Bohr's solar system
model of the atom. Pauli stated that no two electrons
are allowed to be in the same quantum state. This means
that each shell of atomic orbits could only contain a
maximum of two electrons – one with spin up and
the other with spin down. Pauli had quantised space and
he was only 25 years old! From these ideas the periodic
table (proposed by Mendeleev in 1890) could now be explained
from first principles. Pauli eventually won the 1945 Nobel
Prize in Physics for his achievements.