Since the atmosphere is composed of about 78 percent nitrogen,2 a lot of radiocarbon atoms are produced—in total about 16.5 lbs. These rapidly combine with oxygen atoms (the second most abundant element in the atmosphere, at 21 percent) to form carbon dioxide (CO 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.

Radioactive and non-radioactive carbon dioxide mix throughout the atmosphere, and dissolve in the oceans.

radiocarbon dating of wood-35radiocarbon dating of wood-48radiocarbon dating of wood-46

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.

So if half the sand grains are in the top bowl and half in the bottom bowl, then 30 minutes has elapsed since the sand grains began falling.

We can calibrate an hourglass clock by timing the falling sand grains against a mechanical or electronic clock.

After plants and animals perish, however, they no longer replace molecules damaged by radioactive decay.

Instead, the radiocarbon atoms in their bodies slowly decay away, so the ratio of carbon-14 atoms to regular carbon atoms will steadily decrease over time (figure 3).If we assume that the mammoth originally had the same number of carbon-14 atoms in its bones as living animals do today (estimated at one carbon-14 atom for every trillion carbon-12 atoms), then, because we also know the radiocarbon decay rate, we can calculate how long ago the mammoth died. This dating method is also similar to the principle behind an hourglass (figure 4).The sand grains that originally filled the top bowl represent the carbon-14 atoms in the living mammoth just before it died.So even we humans are radioactive because of trace amounts of radiocarbon in our bodies.After radiocarbon forms, the nuclei of the carbon-14 atoms are unstable, so over time they progressively decay back to nuclei of stable nitrogen-14.3 A neutron breaks down to a proton and an electron, and the electron is ejected. The ejected electrons are called beta particles and make up what is called beta radiation. Different carbon-14 atoms revert to nitrogen-14 at different times, which explains why radioactive decay is considered a random process.Knowing the number of atoms that decayed in our sample over a month, we can calculate the radiocarbon decay rate.