An electron is a charged particle so, its
angular momentum from its orbit will result in a magnetic field. This orbital
angular momentum allows it to act like a tiny bar magnet we call spin angular
momentum. Another magnet, the orbital angular momentumn (we refer
to as quantum number L) can interact with the spin angular momentum. .
As long as the orbit (L) does not equal zero (like it would as an s orbital),
coupling between these two magnets can occurs. The interaction of
these two magnets is called spin orbit coupling. There are two possibilities
for these magnets, they can be parallel to each other or anti parallel.
In the parallel position, a higher energy (J) is obtained. We can
think of it as a construcvtive interference. The quantum number l=1 adding
constructively to a spin of 1/2 gives rise to a J value of1-1/2.
If the interaction is destructive, we see l=1 and spin = -1/2 giving rise
to J= 1/2 or a low energy (J). The strength of this coupling increases
with atomic number. Pretend we are "riding on the electron" It will
appear to us the nucleus is orbiting the electron. Nuclei with a high number
of protons will be at the center of a strong magnetic current and we "as
we sit on the electron" will experience a strong magnetic field. 1This
strong field can cause the spin to flip.
Figure 13.17 from Atkins, P. (2001). The
Elements of Physical Chemistry with Applications in Biology, 3rd ed.
W.H.Freeman and Company: New York, p 321.