Pauli exclusion principle

Claimed by Michael Segal

Edited by Ansley Marks

The Pauli exclusion principle is a quantum mechanical principle that asserts that no two electrons in the same atom can occupy the same quantum states simultaneously.

The Main Idea

For each electron in a molecule, there are four distinct values that describe the state of the electron.

Principal Quantum Number (n)

Azimuthal Quantum Number (ℓ)

Magnetic Quantum Number (m)

Spin Quantum Number (s)

The Pauli exclusion principle asserts that all particles are either fermions or bosons. Fermions have an odd multiple of half spins. Bosons have an even multiple of half spins, thus result in an integer amount of spin. For example, helium-3 is a fermion with the spin of 1/2 and helium-4 is a boson with the spin of 0.

The spins have an intrinsic effect on the angular momentum values of a particle. Particles with a half-integer spin (fermions) have antisymmetric states, while particles with a integer spin (bosons) have a symmetric, wavelike functions.

(Left: Fermion; Right: Boson)

History

The Pauli exclusion principle is named after Austrian physicist Wolfgang Pauli. Pauli proposed the assertion in 1925 and received a Nobel Prize in 1945 for his discovery. He is considered a founding father of quantum mechanics and discovered many of his theories and principles without a formal existing definition of what we today call "spin".

Pauli's discovery and implementation of a fourth quantum number laid the groundwork and inspiration for quantum mechanics in the next years following its announcement. Much of Heisenberg's and Shrodinger's discoveries on wave mechanics were built off of the Pauli exclusion principle.

Importance

The significance of the Pauli exclusion Principle is that it introduced a fourth quantum number. While spin is not a physical characteristic, it is crucial when determining shapes and wavelike behaviors of atomic particles. The principle dictates that bosons have probability waves which "flip" as they move and interfere with eachother. The interference of these probability waves leads to collective behavior that can result in lasers, superfluids and superconductors. Conversely, fermions do not flip their probability waves and do not interact with each other.

Sources

Hyper Physics: http://hyperphysics.phy-astr.gsu.edu/hbase/pauli.html

Wikipedia: https://en.wikipedia.org/wiki/Quantum_number

Physics of the Universe: http://www.physicsoftheuniverse.com/topics_quantum_spin.html

APS: http://www.aps.org/publications/apsnews/200701/history.cfm