A radioactive isotope of carbon, whose nucleus contains 6 protons and 8 neutrons; also called → radiocarbon. 14C is naturally produced in the atmosphere when a neutron created by a cosmic ray hits the nucleus of an atom of nitrogen-14. The nucleus absorbs the neutron and ejects a proton, thereby transforming itself into 14C. It decays back to nitrogen, with a half-life is 5730 years, after emitting an electron (146C → 147N + e- + νe). See also → radiocarbon dating.