What defines radioactive elements?

Radioactive elements are defined by their ability to decay or transform over time, which involves the release of energy and particles from the nucleus of the atom. During this decay process, radioactive elements release protons, neutrons, or other particles, which can lead to a change in their atomic number and, consequently, their identity as a different element. This characteristic behavior is central to understanding radioactivity and the implications it has for both the elements themselves and for safety protocols in handling them.

The other options do not accurately represent the nature of radioactive elements. Some elements are stable, meaning they do not undergo decay processes, so the first choice is not applicable. The third option inaccurately states that radioactive elements are always in a liquid state; in reality, they can exist in solid, liquid, or gaseous forms, depending on the specific element and its conditions. Lastly, the notion that radioactive elements exclusively emit only beta particles is incorrect, as they can emit a variety of particles, including alpha particles and gamma radiation, depending on the type of decay they undergo. Thus, the defining feature of radioactive elements is their decay process, which includes the release of protons and changes in atomic structure.