While all the three Hund’s rule is need for the understanding of the fine spectra of atoms, the first rule is sufficient for the electronic configuration of the ground state of the atom.
The first, Hund’s rule of multiplicity used for expressing the electronic configuration of ground state of the atom can be, stated in many ways.
- Stable state of an atom (ground state) has an electronic configuration with maximum electron multiplicity.
- Electrons can occupy the same orbital (multiply), only after filling up of all degenerate orbitals with electrons of parallel spin.
- Electrons, try to have highest multiplicity (unpaired electrons) for stability in side degenerate orbitals
Subshells like p, d, f, have multiple orbitals of same energy (degenerate). For example, p-subshell has three, d -subshell five and f-subshell seven degenerate orbitals.
Though, each orbital can have a maximum of two electrons, Hunds rule of multiplicity, implies that, electrons has to first fill, all the degenerate orbitals available equally before start sharing an another electron in the orbital.
For example, three p-orbitals and five d-orbitals are of same energy. When electrons enter p-subshells, all the three p-orbitals will have one each and only the fourth, fifth and six the electrons can get into sharing the already occupied orbitals.
Similarly, first five electrons enter, all the five d-orbitals and only from the sixth electron sharing the orbitals should take place. The phenomenon is shown, in the picture below.
As, electrons are negatively charged and repel each other, they need more energy to be together in pairing in the same orbital. This increased energy of the electrons as well the atom lead to the destabilization of the atom.
Energy of the electrons and hence that of the atom will decrease when the electrons are nearer to the nucleus. If the electrons in the degenerate orbitals are unpaired, then each electron is attracted and pulled nearer by the nucleus, and stabilizes it by lowering of energy.
If the electrons are also in the same directions, then chances of crossing each other is low and hence the atom will have low energy.