What is the term for atoms that have excess energy and are unstable?

The term for atoms that have excess energy and are unstable is "radioactive." Radioactive atoms are defined by their inability to maintain a stable nucleus due to the presence of excess energy or mass, resulting in their tendency to undergo decay processes. This decay can emit particles or electromagnetic radiation, which is characteristic of radioactive materials.

In contrast, isotopes are variations of elements that contain the same number of protons but different numbers of neutrons; not all isotopes are unstable. Ionized atoms are those that have lost or gained electrons, leading to a positive or negative charge, rather than being directly related to their stability or energy excess. Stable atoms, on the other hand, do not have excess energy and do not undergo radioactive decay. Thus, "radioactive" specifically identifies those atoms that are unstable and have excess energy, making it the correct term.