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How many electrons can fill an s subshell?

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An s subshell can hold a maximum of 2 electrons.1

This limit is determined by the Pauli exclusion principle, which states that an orbital can accommodate no more than two electrons, provided they have opposite spins. Because an s subshell (defined by an orbital angular momentum quantum number of $l = 0$ ) consists of only a single orbital, it is restricted to this two-electron capacity.12