Pauli Exclusion Principle in Atoms

Every electron must have its own unique four-quantum-number identity.

Choose an atom

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SubshellOrbital

Allowed pair

↑ ↓

different spin values

Forbidden pair

↑ ↑

identical quantum state

Shell1

Subshells

Orbital1

Spin↑

Empty atom

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An orbital provides exactly two available spin states.

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An electron state is identified by its shell, subshell, orbital orientation, and spin. No two electrons in the same atom can match in all four.

Electrons in one orbital share the first three quantum numbers. Therefore, their spin values must differ.

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