Today I finish chapter three of George Gamow's classic, Thirty Years That Shook Physics. Chapter 3 is on "W. Pauli and the Exclusion Principle." The previous posts were:
1a. Planck's Quantum
1b. Jumping Photons (Einstein and the Photoelectric Effect)
1c. The Compton Effect (Proof of Energy Packets)
2a. Thomson and Rutherford's Atoms
2b. Bohr's Contributions (How electrons fill the atom)
3a. Pauli Exclusion Principle (no two electrons at any one energy state)
The remainder of chapter 3 deals with modifications to Pauli's exclusion principle and his prediction of the neutrino. Pauli's initial formulation of the exclusion principle was that only two electrons could exist at any one energy state. This proved inadequate because when the lines of the spectra of elements was put through a strong magnetic field, more lines came out than the three components Pauli had come up with could account for.
So it was suggested that an electron had a certain "spin" or "angular momentum" The two electrons might be at the same orbit, but one would have one spin and the other the opposite. This led to the modified Pauli exclusion principle that only one electron could be at each orbit, with the two electrons at roughly the same orbit "spinning" in opposite directions and moving around the nucleus in opposite directions.
The chapter ends with Wolfgang Pauli's suggestion that there might be another particle with no charge and almost no mass. It had seemed like beta particles (i.e., electrons) did not conform to the Law of the Conservation of Energy, another one of the long established principles of physics. For Bohr, what was one more sacred cow. He was willing to throw it out the window as well.
But Pauli suggested that there might be a much smaller particle that could not be detected by charge or mass but that could carry off some of the energy of a beta particle. He called it initially a neutron, but then the neutron with which we are familiar was discovered by Chadwick. Nevertheless, in 1955, the existence of the "neutrino" was confirmed.