Isotopes are variants of an element that have the same number of protons but different numbers of neutrons in their atomic nuclei.
Atomic mass: The total mass of an atom, which is determined by the sum of its protons and neutrons.
Radioactive decay: The process in which unstable isotopes spontaneously break down into more stable forms over time.
Stable isotope: An isotope that does not undergo radioactive decay and remains unchanged over time.