Carbon 14, often written as 14C, is a radioactive form of carbon.
Most carbon 14 is produced in the atmosphere when a free neutron combines with a nitrogen atom to produce a carbon atom. The nitrogen atom had seven protons and seven neutrons. When the additional neutron combines with it, one of the protons is removed, so the atom now has eight neutrons and six protons, making it a form of carbon.
Carbon 14 is unstable. That is, it does not remain in that form, instead eventually decaying back to nitrogen. In any one sample, half of the 14C atoms will decay each 5730 years.
Use in carbon dating
For more information, see Radioactive dating.
14C in the atmosphere is absorbed by plants along with the other forms of carbon (mostly 12C), in the same proportions in which they exist in the atmosphere. The plants are eaten by animals, and so the 14C—and the 14C:12C ratio—is spread throughout the food chain. However, as 14C decays over time, the ratio of 14C and 12C changes, and the amount of change that has occurred is used to determine how long it is since the creature or plant died.