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A very small percentage of carbon, however, consists of the isotope carbon 14, or radiocarbon, which is unstable.Carbon 14 has a half-life of 5,780 years, and is continuously created in Earth's atmosphere through the interaction of nitrogen and gamma rays from outer space.Free 5-day trial Ever wondered how scientists know the age of old bones in an ancient site or how old a scrap of linen is?The technique used is called carbon dating, and in this lesson we will learn what this is and how it is used. Carbon dating, or radiocarbon dating, is a method used to date materials that once exchanged carbon dioxide with the atmosphere. In the late 1940s, an American physical chemist named Willard Libby first developed a method to measure radioactivity of carbon-14, a radioactive isotope. At any given moment, for a certain type of element or a certain type of isotope of an element, there's some probability that one of them will decay. If I wait carbon-14's half-life-- this is a specific isotope of carbon. So when you have the same element with varying number of neutrons, that's an isotope. Let's think about what happens after another half-life. And by the law of large numbers, half of them will have converted into nitrogen-14. This might be the one ultra-stable nucleus that just happened to, kind of, go against the odds and stay carbon-14. But the way we think about half-life is, people have studied carbon and they said, look, if I start off with 10 grams-- if I have just a block of carbon that's 10 grams. Those five grams of carbon-14, every one of those atoms still has, over the next-- whatever that number was, 5,740 years-- after 5,740 years, all of those once again have a 50% chance. Well, after one billion years I'll say, well you know, it'll probably have turned into nitrogen-14 at that point, but I'm not sure. You don't know how well it calibrates against time. This technique works well for materials up to around 50,000 years old.Each radioactive isotope decays by a fixed amount, and this amount is called the half-life.

Because the ratio of carbon 12 to carbon 14 present in all living organisms is the same, and because the decay rate of carbon 14 is constant, the length of time that has passed since an organism has died can be calculated by comparing the ratio of carbon 12 to carbon 14 in its remains to the known ratio in living organisms. Our Living Language : In the late 1940s, American chemist Willard Libby developed a method for determining when the death of an organism had occurred.

1/2 to the 3rd power, because every time you have 1/2 of the original sample-- that's the number of half-lives-- after three half-lives you'll have 1/8 of your original sample. In the next video we're going to explore what if I asked you a question, how many of the particles, or how many grams will you have in exactly 10 days?

And this is just when you're doing it with a discreet you know, when you're right at the half-life point.

Because atmospheric carbon 14 arises at about the same rate that the atom decays, Earth's levels of carbon 14 have remained fairly constant.

Once an organism is dead, however, no new carbon is actively absorbed by its tissues, and its carbon 14 gradually decays.