Hund's first rule

Hund's first rule states that electrons will fill degenerate orbitals (orbitals with the same energy) singly and with parallel spins before pairing up. This means, for example, if there are three orbitals available at the same energy level, electrons will occupy them one each, all with the same spin orientation, rather than pairing up in fewer orbitals. This arrangement minimizes repulsion between electrons and results in a lower energy state. Essentially, electrons prefer to spread out with parallel spins to maximize their stability and reduce electrical repulsion within an atom.