Which of the following best describes a radioactive isotope?

• A. An isotope that is always found in equal abundance as other isotopes of the same element.
• B. An isotope that has the same number of neutrons as protons.
• C. An isotope that spontaneously emits particles or energy to become more stable.
• D. An isotope that does not undergo any nuclear changes over time.

Answer: (C)

Verified step by step guidance

1. Understand what an isotope is: isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
2. Recognize that radioactive isotopes, also called radioisotopes, are unstable isotopes that undergo nuclear decay to reach a more stable state.
3. Recall that during radioactive decay, the isotope spontaneously emits particles (such as alpha or beta particles) or energy (gamma rays).
4. Compare the given options to this definition: the correct description must involve spontaneous emission of particles or energy to become more stable.
5. Conclude that the best description of a radioactive isotope is: 'An isotope that spontaneously emits particles or energy to become more stable.'