Quantum science: Pauli exclusion principal (Introduction)

by David Turell , Monday, January 20, 2025, 19:22 (1 day, 14 hours, 23 min. ago) @ David Turell

From 100 years ago:

https://www.chemistryworld.com/opinion/celebrating-100-years-of-the-pauli-exclusion-pri...

Here he outlined what became known as the exclusion principle, which governs the way electrons become configured in the orbitals of atoms and molecules. Pauli’s idea not only explained the fine details of the electronic spectra of atoms but also ultimately rationalised electron pairing in covalent bonds and made sense of the periodic table. Along with Bohr’s theory of the quantum atom in 1913, the paper might be regarded as establishing a proper quantum description of chemistry.

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In his 1925 paper, he simply accepted the fourth quantum number ms as relating to the magnetic moment of an electron, recognising that it could take the values ±½. The four numbers then wholly characterised an electron’s state. Pauli supposed that as more electrons get added to atoms as the atomic number increases, those already present retain their quantum numbers so that there is a gradual and systematic buildup (Aufbau) of states – an idea already proposed by Bohr. And here was the key: ‘There can never be two or more equivalent electrons in an atom,’ Pauli wrote, ‘for which … the values of all quantum numbers … are the same. If an electron is present in the atom for which these quantum numbers … have definite values, this state is ‘occupied’.’ Given the two-valuedness of ms, this meant that each orbital (as we’d say now) defined by n, l and ml can accommodate just two electrons.

Pauli showed what this implied for the electron configurations of the noble gases, and it was immediately clear that, along with the Aufbau principle, this approach could now account for the form of the entire periodic table. But more than that: Pauli’s exclusion principle, by prohibiting two electrons from sharing a quantum state, also forbids them from occupying the same location in space, and so it produces an effective repulsion much more absolute than that generated by their like charge. It is primarily this that prevents matter from collapsing on itself; in other words, it is what keeps matter stable. Well worth celebrating, I’d say.

Comment: as before we observe that qunatum mechanics is the basis of the universe and all reality.