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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.If we know what fraction of the carbon atoms are radioactive, we can also calculate how many radiocarbon atoms are in the lump.It’s assumed to be the same number of carbon-14 atoms as in elephants living today.With time, those sand grains fell to the bottom bowl, so the new number represents the carbon-14 atoms left in the mammoth skull when we found it.The most well-known of all the radiometric dating methods is radiocarbon dating.Although many people think radiocarbon is used to date rocks, it is limited to dating things that contain carbon and were once alive (fossils).

So one would think that since the radiocarbon dating method works on organic (once-living) materials, then radiocarbon could be used to date fossils.Through photosynthesis carbon dioxide enters plants and algae, bringing radiocarbon into the food chain.Radiocarbon then enters animals as they consume the plants (figure 2).(This 5,730 year period is called the half-life of radiocarbon, figure 5).6 At this decay rate, hardly any carbon-14 atoms will remain after only 57,300 years (or ten half-lives). The decay of radiocarbon follows the exponential decay law, whereby the percentage decrease in the number of parent atoms per unit time is constant.

After each half-life of 5,730 years, the number of parent radiocarbon atoms remaining is halved.

Knowing the number of atoms that decayed in our sample over a month, we can calculate the radiocarbon decay rate.