A unique characteristic of 14C is that it is constantly formed in the atmosphere. Because this decay is constant it can be used as a “clock” to measure elapsed time assuming the starting amount is known. The half-life is the time taken for an amount of a radioactive isotope to decay to half its original value. 14C is radioactive and has a half-life of 5730 years. Hereafter these isotopes will be referred to as 12C, 13C, and 14C. Carbon-12 accounts for ~99.8 % of all carbon atoms, carbon-13 accounts for ~1% of carbon atoms while ~1 in every 1 billion carbon atoms is carbon-14. Three isotopes of carbon are found in nature carbon-12, carbon-13 and carbon-14.