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In living organisms, which are always taking in carbon, the levels of carbon 14 likewise stay constant.

­ ­You probably have seen or read news stories about fascinating ancient artifacts.

At an ar­chaeological dig, a piece of wooden tool is unearthed and the archaeologist finds it to be 5,000 years old.

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.

The number of neutrons, N, is known as the number of neutrons in an atom.