- Spin was first discovered in the context of the emission spectrum of alkali metals. In 1924 Wolfgang Pauli introduced what he called a "two-valued quantum degree of freedom" associated with the electron in the outermost shell. This allowed him to formulate the Pauli exclusion principle, stating that no two electrons can share the same quantum state at the same time.
But the assumption that the inner full shells are spherically symmetric was never justified, and so the necessity of introducing a mysterious two-valued feature of the electron, which had to be accepted even if it was very strange, lacked rationale (but it worked).
The alternative model of this sequence of posts gives evidence of full shells starting from Helium, which are not fully spherically symmetric with a two-valuedness resulting from separation of electrons by repulsion.