Isotopes of carbon. Illustration showing three isotopes of carbon: carbon-12, carbon-13 and carbon-14. Isotopes are forms of an element that contain different numbers of neutrons in the atomic nucleus. Carbon-12 has 6 protons (red) and 6 neutrons (blue) in the nucleus, carbon-13 has 6 protons and 7 neutrons, and carbon-14 has 6 protons and 8 neutrons. All three isotopes also have 6 electrons (black) orbiting the nucleus. Carbon (C) has 15 known isotopes, from carbon-8 to carbon-22, two of which (carbon-12 and carbon-13) are stable. The longest-lived radioisotope is carbon-14, with a half-life of 5,700 years. This is used in archaeology to date biological material.