Pauli's Exclusion Principle: No two electrons in an atom can have all four quantum numbers identical. Two electrons in the same orbital share the same \(n\), \(\ell\), and \(m\) values. The only difference is their spin quantum number \(m_s\) — one has \(+\frac{1}{2}\) (spin up ↑) and the other \(-\frac{1}{2}\) (spin down ↓).