An isotope is a variant of a chemical element that has the same number of protons but a different number of neutrons in its atomic nucleus. This difference in neutrons results in a change in the atomic mass of the element. For example, both Carbon-12 and Carbon-14 are isotopes of Carbon, with Carbon-12 having six neutrons and Carbon-14 having eight neutrons. Isotopes can be stable or unstable. Stable isotopes do not change over time, while unstable isotopes, or radioactive isotopes, decay into other elements or isotopes, releasing energy in the process. This property of radioactive isotopes is utilized in various fields, including medicine for radiotherapy and in archaeology for carbon dating.