The word 'aufbau ' in German menas 'building up' . The building up of orbitals the filling up of orbials with electrons. In the ground state of the atoms, the orbitals are filled in order of is their increasing energies. In unielectron species, energy is determined by 'n' but in multielectron species, energy is determined by `n + l` value .
Answer the following question on the basis of above paragraph :
(a) State Aufbau principle .
(b) Out of 4f and 5s electron orbitals, inbitals, indicate which of higher energy ?

Explanation

(a) N/A
(b) 4f electron orbital has higher energy since its (n+l) value (n+l=4+3=7) is higher than 5s electron orbital (n+l=5+0=5).