In 1922, was proposed the Stern-Gerlach experiment.
Silver atoms are shot through an inhomogeneous magnetic field.
They were either deflected up or down, implying that their angular momentum was quantized.
1922, Otto Stern and Walther Gerlach detect discrete patterns of angular momentum for atoms passing through an inhomogeneous magnetic field. This led to the discovery of electron spin. The proposal that electrons spin about an axis will be proposed 3 years later.