Aufbau principle :
1) In the ground state the electronic configuration can be built up by placing electrons in the lowest available orbitals until the total number of electrons added is equal to atomic number.
2) Thus orbitals are filled in the order of increasing energy.
3) Electrons are assigned to orbitals in order of increasing value of n + l.
4) For sub-shells with the same value of n + l, electrons are assigned first to the sub-shell with lower n