In nature, carbon exists as three isotopes: two stable, nonradioactive (carbon-12 (), and carbon-13 (), and one radioactive carbon-14 (), also known as "radiocarbon"). The half-life of  (the time it takes for half of a given amount of  to decay) is about 5,730 years, so its concentration in the atmosphere might be expected to decrease over thousands of years, but  is constantly being produced in the lower stratosphere and upper troposphere, primarily by galactic cosmic rays, and to a lesser degree by solar cosmic rays. These cosmic rays generate neutrons as they travel through the atmosphere which can strike nitrogen-14 () atoms and turn them into . The following nuclear reaction is the main pathway by which  is created:
n +  →  + p
where n represents a neutron and p represents a proton.
Once produced, the  quickly combines with the oxygen () in the atmosphere to form first carbon monoxide (), and ultimately carbon dioxide ().
Carbon dioxide produced in this way diffuses in the atmosphere, is dissolved in the ocean, and is taken up by plants via photosynthesis. Animals eat the plants, and ultimately the radiocarbon is distributed throughout the biosphere. The ratio of  to  is approximately 1.25 parts of  to 10 parts of . In addition, about 1% of the carbon atoms are of the stable isotope .
The equation for the radioactive decay of  is:
By emitting a beta particle (an electron, e) and an electron antineutrino (), one of the neutrons in the  nucleus changes to a proton and the  nucleus reverts to the stable (non-radioactive) isotope .