Pauli’s Exclusion Principle:
No two electrons in an atom can have all four quantum numbers the same. In other words, an orbital can’t have more than two electrons and moreover, if an orbital has two electrons, they must have opposite spin. Therefore, the capacity of s, p, d, and f subshells to accommodate electrons is 2, 6, 10, and 14 respectively.
Hund’s Rule of Maximum Multiplicity:
According to this rule, the maximum number of unpaired electrons in a given energy level is maximum. In a sub-shell, all the available degenerate orbitals are occupied singly first, and then the pairing of electrons in each orbital occurs. Thus, the 3 degenerate orbitals of p sub-shell (px, py, pz), no one will have 2 electrons as long as any other is vacant. This is known as Hund’s Rule of Maximum Multiplicity.
Orbitals in the same d-sub-shell tend to become completely filled or exactly half-filled with electrons because the half-filled and full-filled d-orbitals have less energy than any other arrangement and are more stable.