Isotopes are variants of a chemical element that have the same number of protons but different numbers of neutrons in their atomic nuclei. This difference in neutrons results in varying atomic masses for the isotopes of the same element. For example, the element carbon has isotopes like carbon-12 and carbon-14, where carbon-12 has six neutrons and carbon-14 has eight. 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 unstable isotopes is utilized in various fields, including radiocarbon dating and nuclear medicine.