Pauli exclusion principle

This explains:

• the rigidity of matter.

• why a star as small as the Sun, when it runs out of nuclear fuel, will not collapse into a black hole.

According to the Pauli-exclusion principle, no two fermions, can occupy the same quantum state, at the same time. It is a theoretical limit on their number per volume, or, spatial density. It is impossible for two particles in the same system to be in the same quantum state at the same time. To explain the electron shells of the periodic table, Pauli proposed that an electron can be defined by 4 quantum numbers:

1. Orbital

2. Shape

3. Inclination

4. Spin

This new property of spin will separate the classification of subatomic particles into 2 families: fermions and bosons. Fermions are all matter particles and have a half-integer spin, while the bosons, are the force carrying particles with an integer value spin.

Wolfgang Pauli proposed his exclusion principle in 1924. This explains some properties of the periodic table, such as how electrons occupy orbits inside the atoms. This would account for how an electron orbital could be full and no longer be able to hold other electrons. More than one electron cannot exist in the same quantum state. This explains why matter has its rigidity and why electrons can't be pushed all the way into the atomic nucleus. Only fermions, the particles with a 1/2 integer value spin obey this principle. The bosons, which have an integer value spin of 1, do not obey this principle and are able to occupy the same quantum state.

Wolfgang Pauli was the first to propose the property of spin, however, did not name it. In 1925, there was a proposal by George Uhlenbeck, Samuel Goudsmit and Ralph Kronig that electrons have spin around their own axis. 

Pauli proposed that spin should be quantized (although he and Heisenberg originally disliked the idea). The electron could either be in a state of "up" or "down" spin.