Q.State and explain Pauli's exclusion principle.

Step 1. Statement: Pauli's exclusion principle states that no two electrons in an atom can have the same set of values for all four quantum numbers - $n$, $l$, $m$ and $s$.

Step 2. Explanation: every electron in an atom must be uniquely identifiable by its own combination of these four numbers; if two electrons happened to share identical $n$, $l$ and $m$ (i.e. occupy the same orbital), the principle forces them to differ in the remaining quantum number, spin - one must be $+\tfrac12$ and the other $-\tfrac12$. …

State the principle precisely (all FOUR quantum numbers), then derive the two …

• Stating the principle loosely as 'two electrons can't be in the same orbital' without the precise all-four-quantum-numbers wording. …