The sequence of filling electrons in sub-shells of elements with few exceptions in…

The sequence of filling electrons in sub-shells of elements with few exceptions in d-block and f-block elements is governed by Aufbau principle followed by Hund’s rule and Pauli’s exclusion principle.
(a ) The electron prefers to enter into sub-shells with lower (n + l) value.
The energy for any sub-shell of an element other than hydrogen is proportional to the sum of principal quantum number (n) and angular momentum quantum number
( b) If (n + l) value is same for many sub-shells, priority of electron filling is given to the sub-shell with lowest n value.
(c ) (i) Fulfilled sub shell is most stable.
(ii) “half filled orbitals are more stable than partially filled orbital”.