The filling of electrons into the orbitals of different atoms take place on the basis of 3 principles :
The electrons are added progressively to Various orbitals in the order of increasing energy, starting with the orbital of lowest energy.
(i) Hydrogen atom: In case of hydrogen atom, energy of orbital is mainly determined by principle quantum number.
For H atom
1s < 2s = 2p < 3s = 3p = 3d < 4s = 4p
(ii) Multi-electronic atoms: In case of multi-electronic atoms, energy of orbitals depends upon both n and l and hence energy of orbitals is compared on the basis of (n + l) rules generally.
⇒ n + l rule:
(iii) As the value (n + l) increases the total energy of orbitals also increases.
Fig: energy levels.
(iv) If value of (n + l) is same or different orbital then the orbital with lower value of n, have lower energy.
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s
While filing the orbitals of same energy (i.e. orbitals of same subshell) pairing of electron start only when each orbital have got 1 electron that too with same spin or parallel spin.
Multiplicity is expressed as 2|s| + 1 where s represent total spin.
2|s| + 1 = 2
, 2|s| + 1 = 2
, 2 + 1 = 4 (It has maximum multiplicity)