Electron Spin
Uhlenbeck & Goudschmidt proposed electron had “intrinsic” angular momentum.
This was the key to understanding the atomic structure of complex atoms.
Notes:
In the late 1920s when they made their discovery, Uhlenbeck and Goudschmidt were two young Dutch physics PhDs. In explaining the origins of their proposal, Sam Goudschmidt says that he often travelled with his uncle, a hardware wholesaler, throughout Europe, where he would visit spectroscopy laboratories. Spectroscopists had noticed curious double lines in some spectra, which they couldn’t understand. He and Uhlenbeck discussed the problem and decided it meant the electron had additional degree of freedom besides moving in three dimensions.
It’s as if the electron acted like a rigid body, but it would violate the uncertainty principle for a quantum mechanical rigid body to have a well-defined velocity of rotation. What’s more, something as small as the electron would have to rotate faster than light speed, precessing as it did so. In fact, it was later shown hat electron spin is a natural consequence of making quantum mechanics relativistic.