As per Hund’s rule, every orbital in a subshell is singly occupied with one electron before any one orbital is doubled occupied, and electrons in singly occupied orbitals have the same spin.
According to Hund’s rule, electron pairing in p, d and f orbitals can’t occur until each orbital of a given subshell contains one electron each or is singly occupied.
Hund’s Formula
One of the electrons must have ms = 1/2
The other must have ms = -1/2
Hund’s first rule states that the lowest energy state is the one that maximises the total spin quantum number for the electron in the open subshell.
Example
Nitrogen's atomic number
is 7. So, its atomic configuration would be
1s² 2s² 2p³
The two 2s electrons will occupy the same orbital, although the three 2p electrons will occupy different orbitals in accordance with Hund’s rule.
Hund’s rule is used in spectroscopy.
Spectroscopy is the study of the interaction between matter and electromagnetic radiation as a function of wavelength.