Concept: Electrons are fermions, and quantum mechanics dictates that fermions cannot occupy the exact same quantum state simultaneously.
Formula: None required.
Solution: - The Pauli Exclusion Principle explicitly formulates this rule for atoms.
- Even if two electrons share the exact same spatial orbital (meaning their \( n \), \( l \), and \( m \) quantum numbers are identical), they must have different spin quantum numbers (\( s \)).
- One must be spin up (\( +1/2 \)) and the other spin down (\( -1/2 \)).
Why other options are incorrect: The Aufbau principle is about filling order from lowest to highest energy. Hund's rule is about maximizing unpaired electrons in degenerate orbitals. Wiswesser's rule relates to line notation in chemical structures, entirely unrelated to quantum numbers.
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