carbon-14 (14C)

A radioactive isotope of carbon with an atomic weight of 14 that is formed in the upper atmosphere by cosmic bombardment. The half-life of carbon-14 is 5,700 years. Radioactive carbon isotopes are incorporated into all organic matter during the lifetimes of plants and animals. Radiocarbon dating of fossil material can therefore be done by measuring the amount of carbon-14 left in samples of, for example, shells or fossil pollens.