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Back in the 1940s, the American chemist Willard Libby used this fact to determine the ages of organisms long dead.Most carbon atoms have six protons and six neutrons in their nuclei and are called carbon 12. But a tiny percentage of carbon is made of carbon 14, or radiocarbon, which has six protons and eight neutrons and is not stable: half of any sample of it decays into other atoms after 5,700 years.Carbon 14 is continually being created in the Earth's atmosphere by the interaction of nitrogen and gamma rays from outer space.Since atmospheric carbon 14 arises at about the same rate that the atom decays, the Earth's levels of carbon 14 have remained constant.When we look at sand in an hourglass, we can estimate how much time has passed based on the amount of sand that has fallen to the bottom.Radioactive rocks offer a similar “clock.” Radioactive atoms, such as uranium (the parent isotopes), decay into stable atoms, such as lead (the daughter isotopes), at a measurable rate.The age of the carbon in the rock is different from that of the carbon in the air and makes carbon dating data for those organisms inaccurate under the assumptions normally used for carbon dating.This restriction extends to animals that consume seafood in their diet.

This rules out carbon dating for most aquatic organisms, because they often obtain at least some of their carbon from dissolved carbonate rock.

Yet this view is based on a misunderstanding of how radiometric dating works.

Part 1 (in the previous issue) explained how scientists observe unstable atoms changing into stable atoms in the present.

The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb.

What makes this fact useful is that they occur at different rates, as expressed in their half-lives (the time it takes for half the atoms to decay).