Fermions in Particle Physics -(Pauli Exclusion Principle) #Physik_World

Fermions: Fermions are those particles which includes quarks and leptons. Quarks and leptons are spin -1/2  particles, implying that they are fermions according to the spin-statistics theorem. Fermions follows Fermi-Dirac statistics. These particles obey the Pauli exclusion principle.

The Pauli exclusion principle is the quantum mechanical principle which states that two or more identical fermions (particle with half integer spin) cannot occupy the same quantum state within a quantum system simultaneously. In the case of electrons in atoms, it can be stated as follows:

• n, the principle quantum number,

• l, the angular momentum,

• _{ml, themagnetic quantum number and}

• _{ms, the spin quantum number.}

For example, if two electron reside in the same orbital, and if their n, l and m_{l value are same, then their} m_{s must be different and thus the electrons must have opposite half-integer spin projection of 1/2 and -1/2. (Spin Up and Down)} 

Note: Three of quantum numbers can be same but not all; here spin quantum number is different according to the Pauli Exclusion Principle. Hence Fermions have half integer spin.