State Hund’s rule. What is the reasonbehind this rule ?

Hund's rule states that when filling orbitals of equal energy (degenerate orbitals), electrons will occupy separate orbitals with parallel spins before pairing up.

The reason behind this rule is based on the principle of electron-electron repulsion. Electrons have a negative charge and therefore repel each other. By occupying separate orbitals with parallel spins, the electrons are able to maximize their distance from each other, reducing the repulsion between them. This arrangement results in a lower overall energy for the system, making it more stable.

In summary, Hund's rule ensures that electrons are distributed in a way that minimizes electron-electron repulsion and maximizes the stability of the system.