Pauli Exclusion Principle

In the early twentieth century, the problem concerning the number and the arrangement of bound electrons in an atom became a central aspect for both physicists and chemists. In this chapter, we will start from the discussion of the works of J. Rydberg, G. Lewis and I. Langmuir, aimed at explaining the periodic table of elements. Then, we will deal with the splitting of atomic spectral lines by a magnetic field (namely, the Zeeman effect), which was discovered by P. Zeeman in Leiden, and, subsequently, interpreted by A. Lorentz. In particular, we will focus on the anomalous Zeeman effect that, in 1925, led W. Pauli to the formulation of the so-called exclusion principle and to the idea of electronic spin, first introduced by S. Goudsmit and G. Uhlenbeck. Finally, we will return to Leiden, to briefly dwell with the experiments at very low temperatures conducted by H. Kamerlingh Onnes.