Pauli's exclusion principle states that no two electrons in an atom can have the same set of all the four quantum numbers. Aufbau principle: It states that electrons are added progressively to the various orbitals in their order of increasing energy starting with the orbitals of lowest energy. Hund's rule: It states that electron pairing in any orbital (s,p,d,f) cannot take place until each orbital of the same sub-level contain 1-electron.