The Pauli exclusion principle states that

  • A. no two electrons in an atom can have the same set of all four quantum numbers
  • B. electrons always pair up
  • C. orbitals fill in order of increasing energy
  • D. electrons occupy the nucleus

Explanation

Because the first three quantum numbers define an orbital, the principle limits any orbital to two electrons, which must then differ in spin. This single rule is what gives every shell its fixed capacity and ultimately why the periodic table has the shape it does. The filling order is the separate aufbau principle.

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