Which of the following best describes a radioactive isotope?

A radioactive isotope is defined as an unstable atom that undergoes radioactive decay, during which it emits radiation in the form of particles or electromagnetic waves. This instability arises from an imbalance in the forces holding the atom together, often due to an excess of neutrons or a deficiency of protons. As a result, the nucleus changes over time, leading to the emission of radiation, which can include alpha particles, beta particles, or gamma rays. This process is a defining characteristic of radioactive isotopes and is crucial for applications in medical imaging, cancer treatment, and radiometric dating among others.

The other descriptions do not capture the essence of what makes an isotope radioactive. Stable atoms, equal numbers of protons and neutrons, or those that are simply abundant in nature do not inherently reflect the unstable nature and radiation-emitting properties that characterize radioactive isotopes.