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CHEM 121 Studioby Learn4Less · UBC CHEM 121

CHEM 121 glossary

Pauli exclusion principle

No two electrons in an atom can have the same four quantum numbers. So an orbital holds at most two electrons, and they must have opposite spins (ms=+12m_s = +\tfrac12 and −12-\tfrac12).

Helium: two electrons in 1s1s, spins opposite.

Example

Helium's two electrons are (1,0,0,+12)(1, 0, 0, +\tfrac12) and (1,0,0,−12)(1, 0, 0, -\tfrac12). 1s1s is then full, so lithium's third electron must go into 2s2s.

See alsoSpin quantum numberQuantum numbersAufbau principleHund's rule