¹The term isotope refers to differences in atomic mass for any chemical element. For example, the most common isotope of the element carbon (C) has six neutrons and six protons within each carbon atom nucleus, giving that isotope an atomic mass of twelve (¹²C). A carbon atom having two more neutrons in its nucleus would be an example of the isotope ¹⁴C, which just happens to be radioactive: the nucleus is unstable, and will over time decay, emitting energy and particles and in the process change into another element.