Hund's second rule

Hund's second rule states that, when electrons occupy equal-energy orbitals, they tend to pair up only after each orbital has at least one electron with parallel spins. In other words, electrons prefer to stay unpaired in different orbitals with the same spin, rather than pairing up prematurely. This arrangement maximizes the total spin, which leads to a lower overall energy of the atom, making the atom more stable. Essentially, electrons fill orbitals in a way that keeps their spins aligned as much as possible before pairing occurs.