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Radioactive Dating D.3.1 Outline the method for dating rocks and fossils using radioisotopes, with reference to 14C and 40K.

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***The limit for accurate determination of age is about 50,000yrs***

D.3.1 Outline the method for dating rocks and fossils using radioisotopes, with reference to 14C and 40K. 14C is a naturally occurring isotope of carbon with a half life of 5730 years; It is constantly being made in the atmosphere when cosmic rays cause neutrons to fuse with nitrogen nuclei and “kick out” protons Decay Formation 14C production is in equilibrium with its decay to 12C The 14C is incorporated in carbon dioxide which is then taken up by plants. In the end all living things have the same ratio of 14C to 12C When an organism dies it no longer takes in 14C. So over time the ratio of 14C to 12C changes. This is measurable and can be used to estimate age. *The limit for accurate determination of age is about 50,000yrs*

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**40K is an isotope with a half life of 1.3 X 109 yrs**

It decays to 40Ar. When 40K is released from a volcano in lava all of the argon gas is driven off. So brand new rocks effectively have a ratio 40K: 40Ar (or 100:0) Over time the lava may be weathered and eroded and incorporated into sedimentary rocks. *The measured ratio of 40K to 40Ar can be used to date rocks over one million years old with an accuracy of around 50,000 years*

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**The time it takes for half of a radioactive isotope to decay**

D.3.2 Define half-life. The time it takes for half of a radioactive isotope to decay Simulation of many identical atoms undergoing radioactive decay, starting with either four atoms (left) or 400 atoms (right). The number at the top indicates how many half-lives have elapsed

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What is the half life for each of the isotopes represented by these curves?

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3.42 0.13 0.7 ?.??

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**D.3.3 Deduce the approximate age of materials based on a simple decay curve for a radioisotope.**

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