Why do most molecules contain electrons in pairs?

Most molecules contain electrons in pairs primarily due to the Pauli exclusion principle, a fundamental quantum mechanical requirement. This principle dictates that no more than two electrons can occupy a single atomic orbital, and if two electrons do occupy the same orbital, their spin must be paired—meaning one must have an "up" spin and the other a "down" spin.1

In the formation of covalent bonds, atoms share electrons to achieve a lower, more stable energy state. These shared electrons typically occupy a bonding molecular orbital as a pair. Because the formation of these electron pairs is energetically favorable, most stable molecules consist of even numbers of electrons.23

In contrast, a radical is a molecule that contains at least one unpaired electron. Because of this unpaired electron, radicals are generally highly reactive and tend to seek out other atoms or molecules to complete their electron pairs, thereby returning to a more stable, paired state.3