**Pauli Exclusion Principle** Because of the quantum mechanical Uncertainty Principle, quantum mechanics methods treat electrons as indistinguishable particles. This leads to the Pauli Exclusion Principle, which states that the many-electron wave function—which depends on the coordinates of all the electrons—must change sign whenever two electrons interchange positions. That is, the wave function must be antisymmetric with respect to pair-wise permutations of the electron coordinates.

The Exclusion Principle is quantum mechanical in nature, and outside the realm of everyday, "classical" experience. Think of it as the inherent tendency of electrons to stay away from one another; to be mutually excluded. Exclusion is due to the antisymmetry of the wave function and not to electrostatic coulomb repulsion between two electrons. Exclusion exists even in the absence of electrostatic repulsions.

Quotation from j.chem.edu Gillespie (2004):

**Quote:**

...electrons with the same spin have a zero probability of being found at the same location, and an increasing probability of being found at an increasing distance apart.

It can, perhaps, be useful to think of opposite spin α and β (or spin-up and spin-down) electrons as existing in two independent “worlds” (two independent, but overlapping, sets of three dimensions) in which, if we ignore their mutual electrostatic repulsion, the α electrons do not “see” the β electrons in the other “world” and so they have no influence on each other’s behavior, whereas electrons of the same spin in the same “world” have to share the space available to them andthus, keep as far apart as possible.

Gillespie, "Chemical Bonding and Molecular Geometry: From Lewis to Electron Densities, 2001, p74

**Quote:** Electrons with the same spin have a zero probability of being found simultaneously at the same point in space, a low probability of being found close together, and are most probably to be found as far apart as possible.

__See also:__ Pauli Blocking

http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1342027318/0#0