# Why don't all the electrons in an atom fall to the lowest energy level?

Look at "applied to atoms". More specifically, no two electrons may have identical quantum numbers $n , l , {m}_{l} ,$ and ${m}_{s}$. Since ${m}_{s}$ may only be equal to $- \frac{1}{2} ,$ or$\frac{1}{2}$, this limits the variety of electrons per subshell per quantum number per magnetic quantum number to 2.