### quantum number

(number representing a conserved quantity)

A **quantum number** is a number used in quantum mechanics
that represents an amount of some quality that is preserved
through interactions between particles, e.g., if two particles
combine, then their resulting particle has the sum of their
individual quantum numbers representing that quality. Atoms and
subatomic particles have quantum numbers of different types
(i.e., for different such qualities) and through the interaction
of particles, including splitting or combining, the sum of each
type of quantum number remains the same. This is analogous to the
physics concept of the conservation of energy: that total energy
remains the same through an interaction, but quantum numbers deal
with finer detail.

There are a number of "well known" quantum numbers associated with
normal matter, electrons, protons, neutrons, and atoms, but those
are for the quantum mechanical description of interactions of
interest.

Examples of interactions in which quantum number conservation is
of interest include emission of a photon by an atom whose
electrons have a more-than-minimal state of excitation, the
analogous absorption, photoionization, **fission**,
fusion, and beta decay.

Examples of quantum numbers are spin and electric **charge**.
Quantum numbers are generally specified so as to always be
integral, though spin is defined such that the smallest unit
is termed 1/2. (See "electron orbital" for more regarding
quantum numbers associated with electron orbits.)

(*quantum mechanics,physics*)
http://en.wikipedia.org/wiki/Quantum_number

**Referenced by:**

antimatter

conservation law

electron orbital

electron shell

neutralino

particle

Pauli exclusion principle

quantum mechanics (QM)

spin (m_{s})

supersymmetry (SUSY)

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