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If carbon-14 has formed at a constant rate for a very long time and continually mixed into the biosphere, then the level of carbon-14 in the atmosphere should remain constant.
This carbon dioxide, now radioactive with carbon-14, is otherwise chemically indistinguishable from the normal carbon dioxide in the atmosphere, which is slightly lighter because it contains normal carbon-12.
Since each beta particle represents one decayed carbon-14 atom, we know how many carbon-14 atoms decay during a month.
Chemists have already determined how many atoms are in a given mass of each element, such as carbon.4 So if we weigh a lump of carbon, we can calculate how many carbon atoms are in it.
The reason is that, as long as the organism is alive, it replaces any carbon molecule that has decayed into nitrogen.
After plants and animals perish, however, they no longer replace molecules damaged by radiocarbon decay.
The radiocarbon half-life or decay rate has been determined at 5,730 years.