Q.Using Hund's rule, draw the orbital (box) diagram for the $2p$ subshell of nitrogen ($Z = 7$, configuration $1s^2\,2s^2\,2p^3$).

Together with the Aufbau principle, two further rules govern exactly how electrons occupy the orbitals of a many-electron atom in its ground state.

The Pauli exclusion principle. Proposed by Wolfgang Pauli in 1925, this principle states that no two electrons in the same atom can have an identical set of all four quantum numbers ($n$, $l$, $m_l$, $m_s$). Since a particular combination of $n$, $l$ and $m_l$ already specifies one orbital, and the spin quantum number $m_s$ can only take one of two values ($+\tfrac{1}{2}$ or $-\tfrac{1}{2}$), this principle has a direct and very useful consequence: any one orbital can hold a maximum of two electrons, and if it holds two, their spins must be paired (opposite) -- one $+\tfrac{1}{2}$ and one $-\tfrac{1}{2}$. This single rule fixes the maximum electron capacity of every subshell (2 for s, 6 for p, 10 for d, 14 for f) and, by extension, of every shell ($2n^2$). …