Question

What is Pauli's Exclusion Principle?

Answer 1

The Pauli Exclusion Principle states that no two electrons may occupy the same quantum state. If they do, the total wave function must vanish.

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When two electrons are in the same orbital, they must share all BUT ONE quantum number, and four are required to specify a quantum state. The remaining one is the spin quantum number, `m_s = pm1/2`.

So we then have that this configuration is allowed:

`ul(uarr darr)`

For this, `m_s(1) = +1/2` and `m_s(2) = -1/2`.

And this configuration is forbidden:

`ul(uarr uarr)`

For this, `m_s(1) = +1/2` and `m_s(2) = +1/2`...