In the 19th century, Henri Becquerel discovered that some chemical elements have atoms that change: During their change they send out a particle. In 1898, Marie and Pierre Curie called this phenomenon radioactive decay. Becquerel and the Curies were awarded the Nobel Prize in Physics for this discovery, in 1903.
Most carbon atoms have six protons and six neutrons in their nucleus. This carbon is called carbon-12, because 12 is the number of protons plus the number of neutrons in the carbon-12 nucleus (six protons + six neutrons = 12). Its atomic weight is 12. If a carbon atom has two more neutrons it is carbon-14. Carbon-14 acts chemically like other carbon, because the six protons and six electrons are what govern its chemical properties. In fact, carbon-14 exists in all living things; all plants and animals contain carbon-14. However, carbon-14 is radioactive. It decays by beta decay. It can be detected and measured. Carbon-14, in the small amounts found about us in nature, is harmless. In archeology, this kind of carbon is used to determine the age of wood and other formerly living things. The method is called Radiocarbon dating which is one kind of radiometric dating.