The stable form of carbon is carbon 12 and the radioactive isotope carbon 14 decays over time into nitrogen 14 and other particles.
Carbon is naturally in all living organisms and is replenished in the tissues by eating other organisms or by breathing air that contains carbon.
Once an organism dies, it stops taking in carbon-14.The carbon-14 it contained at the time of death decays over a long period of time, and the radioactivity of the material decreases.Materials that originally came from living things, such as wood and natural fibres, can be dated by measuring the amount of carbon-14 they contain.For example, in 1991, two hikers discovered a mummified man, preserved for centuries in the ice on an alpine mountain.The half-life of a radioactive isotope describes the amount of time that it takes half of the isotope in a sample to decay.
In the case of radiocarbon dating, the half-life of carbon 14 is 5,730 years.
Experts can compare the ratio of carbon 12 to carbon 14 in dead material to the ratio when the organism was alive to estimate the date of its death.
Radiocarbon dating can be used on samples of bone, cloth, wood and plant fibers.
Libby, a Professor of Chemistry at the University of Chicago, predicted that a radioactive isotope of carbon, known as carbon-14, would be found to occur in nature.
Working with several collaboraters, Libby established the natural occurrence of radiocarbon by detecting its radioactivity in methane from the Baltimore sewer.
It takes about 5,730 years for half of a sample of radiocarbon to decay back into nitrogen.