What is Spin?

Spin is a given property  of particles (electrons, protons, neutrons, etc.) such as mass or charge. Spin is significant for the structure of atoms and molecules and forms the basis of magnetism and of many chemical phenomena. There are two comparisons to classical physics that illustrate spin properties:

1. Particles as tiny spinning tops

In classical physics, a so-called angular momentum can be attributed to any object that rotates. This depends on the mass and shape of the body and the speed at which it rotates. The spin also represents an angular momentum. In a model, small particles such as electrons can be described as globules rotating around their own axis. This idea is useful in explaining the behavior of particles; however, it does not correspond to measurable reality.

2. Particles as tiny magnets

A magnetic field is associated with the spin. You can imagine the particles in the model as tiny bar magnets with a north and a south pole. Even if the magnetic fields of bar magnets and particles with spin have the same shape, there is an important difference in their behavior: bar magnets can rotate in any direction, but the orientation of quantum mechanical magnetic moments is “quantized”: the spin of electrons, for example, can take only exactly two orientations in an external magnetic field, denoted "spin up" and "spin down" (for experimental evidence see „The Stern-Gerlach experiment").


The Stern-Gerlach experiment