Isotopes (from Greek Ισότοπο, "same place") are forms of chemical elements which have the same atomic number but a different atomic mass, or in other words the same number of protons in the atomic nucleus but different numbers of neutrons. Radioactive isotopes are unstable forms of chemical elements that break down into smaller elements and release radiation in the process as some mass is converted into energy.
For example, carbon-12 (12C), carbon-13 (13C), and carbon-14 (14C) are three isotopes of carbon, each containing six protons and also containing six, seven, or eight neutrons, respectively. While carbon-12 is the most common form, 13C is magnetically active, and it is therefore useful for magnetic resonance imaging (MRI) and nuclear magnetic resonance spectroscopy. Carbon-14 is radioactive, and is therefore useful for radiation tracing and age determination.