Electrons filling quantum energy levels
When a state has only one electron, it could be either spin-up or spin-down. However, according the the Pauli Exclusion Principle, when there are two in a state, there must be one of each.
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"Materials in Electronics/The Aufbau Principle."
http://en.wikibooks.org/wiki/Materials_in_Electronics/The_Aufbau_Principle
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