Question

According to the Pauli exclusion principle, when can two electrons occupy the same orbital?

Answer 1

When they have the opposite spins.

---

The Pauli Exclusion Principle actually states that two electrons cannot occupy the same quantum state. To occupy the same quantum state, they would have the same `n` (quantum level), `l` (subshell), `m_l` (orbital), and `m_s` (spin).

In order to differ in quantum state, but be in the same orbital (same `m_l`), you only have to have a different `m_s`. Since `m_s` can only be `+1/2` or `-1/2`, the electrons just have to be opposite spins to be in the same orbital.