Definition of Hund’s Rule
Hund’s rule can be stated in a way that every orbital present in the sublevel of an atom is first singly occupied by an electron. When all the orbitals present in the atom are occupied by single electrons, then they are doubly filled. It is because electrons carry a negative charge and hence, repel each other, therefore, they tend to remain in their orbitals to minimize repulsion.
Moreover, the electrons present in the singly occupied orbitals must have the same spin (either clockwise or anti-clockwise). In this way, the total spin is incredibly maximized. The direction of the spin of electrons of all orbitals is dependent on the spin of the first electron. Following is an example given of nitrogen orbitals: