Carbon-14 is a radioactive isotope of carbon. From "
Carbon: Properties and Isotopes"(1):
Carbon has 13 known isotopes, which have from 2 to 14 neutrons in the nucleus and mass numbers from 8 to 20. Carbon-12 was chosen by IUPAC in 1961 as the basis for atomic weights; it is assigned an atomic mass of exactly 12 atomic mass units. Carbon-13 absorbs radio waves and is used in nuclear magnetic resonance spectrometry to study organic compounds. Carbon-14, which has a half-life of 5,730 years, is a naturally occurring isotope that can also be produced in a nuclear reactor.
The amount of carbon-14 in the atmosphere is normally very low compared to the amounts of carbon-12 and carbon-13 (both stable isotopes). From "
The 14C Method"(7):
Three principal isotopes of carbon occur naturally - C-12, C-13 (both stable) and C-14 (unstable or radioactive). These isotopes are present in the following amounts C12 - 98.89%, C13 - 1.11% and C14 - 0.00000000010%.
This atmospheric carbon-14 is produced by cosmic ray bombardment. From "
How Carbon-14 Dating Works"(5):
Cosmic rays enter the earth's atmosphere in large numbers every day. For example, every person is hit by about half a million cosmic rays every hour. It is not uncommon for a cosmic ray to collide with an atom in the atmosphere, creating a secondary cosmic ray in the form of an energetic neutron, and for these energetic neutrons to collide with nitrogen atoms. When the neutron collides, a nitrogen-14 (seven protons, seven neutrons) atom turns into a carbon-14 atom (six protons, eight neutrons) and a hydrogen atom (one proton, zero neutrons). Carbon-14 is radioactive, with a half-life of about 5,700 years.
This takes energy to accomplish, and the decay releases this energy: carbon-14 decays back to Nitrogen-14 by beta- decay. From "
Glossary: Beta Decay"(9):