Aufbau principle :
- 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.
- Thus orbitals are filled in the order of increasing energy.
- Electrons are assigned to orbitals in order of increasing value of n + l.
- For sub-shells with the same value of n + l, electrons are assigned first to the sub-shell with lower n