When are atoms most stable?

Atoms are most stable when all of the electron orbitals in the valence shell are filled. The stability of an atom is largely determined by its electron configuration, particularly in the outermost shell, known as the valence shell. Atoms strive to achieve a full valence shell because this arrangement is energetically favorable and leads to lower potential energy.

Atoms with filled valence shells have the maximum number of electrons allowed in that shell, which corresponds to the octet rule for many elements, especially those in the second and third periods of the periodic table. A complete valence shell typically leads to chemical inertness, as seen in noble gases, which do not readily form compounds with other elements.

When atoms have incomplete valence shells, they tend to be more reactive, seeking to lose, gain, or share electrons with other atoms to achieve a filled shell. Therefore, the condition of having all electron orbitals in the valence shell filled is what leads to the maximum stability of an atom.