What is the state called when an atom is stable?

The state of an atom is considered to be stable when it is in equilibrium. In this context, equilibrium refers to a situation where the forces acting upon the atom are balanced and its electrons are in their lowest energy levels. This stable arrangement allows the atom to maintain its structure without easily forming bonds with other atoms or losing electrons.

In contrast, the excited state represents a situation where an atom has absorbed energy and its electrons jump to higher energy levels, making it more reactive. The reactive state indicates that the atom is likely to engage in chemical reactions, typically because it seeks to achieve stability by bonding with other atoms or gaining or losing electrons. An ionized state signifies that an atom has either lost or gained electrons, resulting in a charged particle which is inherently unstable compared to its neutral form. Thus, equilibrium is the correct term for the stable state of an atom.