11.2. Radioactive Decay http://www.ck12.org
MEDIA
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URL: http://www.ck12.org/flx/render/embeddedobject/5017How Carbon-14 Dating Works
Carbon-14 forms naturally in Earth’s atmosphere when cosmic rays strike atoms of nitrogen-14. Living things take
in and use carbon-14, just as they do carbon-12. The carbon-14 in living things gradually decays to nitrogen-14.
However, it is constantly replaced because living things keep taking in carbon-14. As a result, there is a fixed ratio
of carbon-14 to carbon-12 in organisms as long as they are alive. This is illustrated in the top part ofFigure11.10.
After organisms die, the carbon-14 they already contain continues to decay, but it is no longer replaced (see bottom
part ofFigure11.10). Therefore, the carbon-14 in a dead organism constantly declines at a fixed rate equal to the
half-life of carbon-14. Half of the remaining carbon-14 decays every 5,730 years. If you measure how much carbon-
14 is left in a fossil, you can determine how many half-lives (and how many years) have passed since the organism
died.
FIGURE 11.10
After organisms die, the carbon-14 they
contain is lost at a constant rate.