Quantum Contradictions 13: Why Must the Electron Spin?

Wikipedia informs us:

• 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.

The objective was to explain the observed doublet feature of the emission spectra of alkali metals with one valence electron in the outermost shell, from an assumption that the inner full shells (and kernel) are fully spherically symmetric. The only possibility left was then to equip the single outer valence electron with a Zweideutigkeit or two-valuedness, which was later named spin up and spin down. The same argument was used to explain the Stern-Gerlach experiment.

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.