For example carbon has three naturally occuring isotopes called carbon 12, carbon, 13, and carbon 14 (the 12,13,14 are the mass numbers) they are all carbon atoms because they contain 6 protons and have the atomic number of 6 but a different mass number. Carbon 12 and carbon 13 are stable isotopes of carbon but carbon 14 is unstable and therefore radioactive when an element undergoes radioactive decay the rearrangements of protons and neutrons can cause the nuclei to emit high-energy