Hund’s Rule Definition:
The Hund’s rule states, when assigning electrons to an atom, one electron is placed in each degenerate orbital first, before they are paired.
Hund’s Rule Explained:
To visualize the Hund’s rule of electron assignment let’s consider the atoms Carbon, Nitrogen and Oxygen. Each of these atoms has 6,7, and 8 electrons respectively that occupy orbitals 1s, 2s, and 2p. You should focus on the assignment in the degenerate p-orbitals as we move from Carbon towards Oxygen.