The stability of an atom generally increases when which condition is met?

An atom achieves stability when its outer electron shell is full, which typically means that it contains eight electrons. This concept is based on the octet rule, which states that atoms are most stable when they have a complete set of eight valence electrons in their outer shell. Atoms strive to reach this stable configuration either by gaining, losing, or sharing electrons through chemical bonding.

When an atom has eight electrons in its outer shell, it is less likely to react with other atoms, making it chemically inert, as seen in the noble gases. This full outer shell provides a lower energy state for the atom, contributing to its overall stability.

In contrast, having fewer than eight electrons in the outer shell typically makes an atom more reactive, as it will seek to gain or lose electrons to achieve a full outer shell. Thus, the correct answer reflects the condition under which atoms are usually most stable.