Pauli’s Exclusion Principle:
According to Pauli’s exclusion principle, “No two electrons in an atom can have the same set of all four quantum numbers”.
It follows, therefore, that maximum of two electrons can be accommodated in an orbital and they must possess opposite spins.
Aufbau principle:
The Aufbau principle states that “In the ground state of an atom, an electron enters the orbital of lowest energy first and subsequent electrons are fed in the order of increasing energies”.
From an energy level diagram for multi-electron atoms, the following sequence observed for orbitals in the increasing order of energy:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s
According to Aufbau principle, the orbital should be filled in the above sequence. It is very important to remember that the sequence of energy levels pertains up to 3p and then 4s-orbital comes first instead of 3d. In fact, the energy of an orbital is determined by the quantum numbers n and l with the help of important rule known as (n + 1) rule or Bohr Bury’s rule
Hund’s Rule of Maximum Multiplicity:
According to this rule, “Electron pairing will not take place in orbitals of same energy (same suhshell) until each orbital is singly filled”.
This suggests that it is difficult for an electron to enter an orbital which already has an electron than to enter an unoccupied orbital of same energy. This principle is very important in guiding the filling of p, d and f orbitals, which have more than one kind of orbitals. For example, we know that there are three p-orbitals (px, py and pz) of the p-subshell in a principal energy level. According to Hund’s rule, each of the three p-orbitals must get one electron of parallel spin before any one of them receives the second electron of opposite spin.