Editing Spin quantum number (section)

== Detection of spin ==
When lines of the hydrogen spectrum are examined at very high resolution, they are found to be closely spaced doublets. This splitting is called fine structure, and was one of the first experimental evidences for electron spin. The direct observation of the electron's intrinsic angular momentum was achieved in the [[Stern–Gerlach experiment]].

=== Stern–Gerlach experiment ===
{{main | Stern–Gerlach experiment}}
The theory of spatial quantization of the spin moment of the momentum of electrons of atoms situated in the [[magnetic field]] needed to be proved experimentally. In [[1922 in science|1922]] (two years before the theoretical description of the spin was created) [[Otto Stern]] and [[Walter Gerlach]] observed it in the experiment they conducted.

[[Silver]] atoms were evaporated using an electric furnace in a vacuum. Using thin slits, the atoms were guided into a flat beam and the beam sent through an in-homogeneous magnetic field before colliding with a metallic plate. The laws of classical  physics predict that the collection of condensed silver atoms on the plate should form a thin solid line in the same shape as the original beam. However, the in-homogeneous magnetic field caused the beam to split in two separate directions, creating two lines on the metallic plate.

The phenomenon can be explained with the spatial quantization of the spin moment of momentum. In atoms the electrons are paired such that one spins upward and one downward, neutralizing the effect of their spin on the action of the atom as a whole. But in the valence shell of silver atoms, there is a single electron whose spin remains unbalanced.

The unbalanced spin creates [[spin magnetic moment]], making the electron act like a very small magnet. As the atoms pass through the in-homogeneous magnetic field, the [[force moment]] in the magnetic field influences the electron's dipole until its position matches the direction of the stronger field. The atom would then be pulled toward or away from the stronger magnetic field a specific amount, depending on the value of the valence electron's spin. When the spin of the electron is {{sfrac|+| 1 |2}} the atom moves away from the stronger field, and when the spin is {{sfrac|−| 1 |2}} the atom moves toward it. Thus the beam of silver atoms is split while traveling through the in-homogeneous magnetic field, according to the spin of each atom's valence electron.

In [[1927 in science|1927]] Phipps and Taylor conducted a similar experiment, using atoms of [[hydrogen]] with similar results.  Later scientists conducted experiments using other atoms that have only one electron in their valence shell: ([[copper]], [[gold]], [[sodium]], [[potassium]]). Every time there were two lines formed on the metallic plate.

The [[atomic nucleus]] also may have spin, but protons and neutrons are much heavier than electrons (about 1836 times), and the magnetic dipole moment is inversely proportional to the mass. So the nuclear magnetic dipole momentum is much smaller than that of the whole atom. This small magnetic dipole was later measured by Stern, Frisch and Easterman.

===Electron paramagnetic resonance===
For atoms or molecules with an unpaired electron, transitions in a magnetic field can also be observed in which only the spin quantum number changes, without change in the electron orbital or the other quantum numbers. This is the method of [[electron paramagnetic resonance]] (EPR) or electron spin resonance (ESR), used to study [[Radical (chemistry)|free radicals]]. Since only the magnetic interaction of the spin changes, the energy change is much smaller than for transitions between orbitals, and the spectra are observed in the [[microwave]] region.