☛ a. Aufbau Principle: According to this principle, In the ground state of an atom the electrons are added one by one to the various orbitals in order of their increasing energy starting with the orbitals of lowest energy. The order of increasing energies of various orbitals can be calculated by the (n+1) rule. However if the (n+1) value of two different orbitals are same then the orbitals with lower value of n has lower energy.

☛ b. Pauli Exclusion Principle: This principle states that an orbital can have maximum two electrons and these must have opposite spins.

☛ c. Hund’s rule of maximum multiplicity: Electron pairing in p, d and f orbitals cannot occur until each orbital of a given subshell contains one electron or is singly occupied. This happens because electrons being identical in charge repel each other when present at the same orbital.