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It can be difficult to determine the ages of objects by sight alone. e.g. It can be difficult to tell which students in a classroom are oldest. Radioactivity provides a method to determine age by comparing the relative amount of remaining radioactive material to the amount of stable products formed.

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Remember from our look at compounds, that most living organisms are based on organic molecules (i.e. contain carbon). Carbon has two main forms: a stable carbon-12 isotope and a radioactive isotope, carbon-14, and these exist naturally in a constant ratio. In nature, carbon-12 appears 98.9% of the time, while one carbon-14 atom appears for every 1 trillion normal atoms.

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When an organism dies, carbon-14 stops being created and slowly decays. Measuring the relative amounts of carbon-12 : carbon-14 is called radiocarbon dating. What we find is that half of a sample of carbon-14 every 5730 years, regardless of the original size of the samplethis is the half-life of carbon-14. So radiocarbon dating can be used to provide the age of any organism or organic material… l ess than 50 000 years old (… more on why later!) Using radiocarbon dating, these cave paintings of horses, from France, were determined to have been drawn 30 000 years ago.

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Half-life = time required for half of a radioactive sample to decay. The half life for a radioactive element is a constant rate of decay. e.g. Strontium-90 has a half-life of 29 years. If you have 10 g of strontium-90 today, there will be 5 g remaining in 29 years. Half-lives of many common radioisotopes are listed on pg.12 of your data booklet.

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Parent isotope = the original, radioactive material. Daughter isotope = the stable product of the radioactive decay. The rate of decay remains constant, but some elements require one step to decay, while others decay over many steps before reaching a stable daughter isotope. Carbon-14 decays into nitrogen-14 in one step Uranium-235 decays into lead-207 in fifteen steps. Thorium-235 decays into lead-208 in ten steps.

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After 1 st half-life After 2 nd half-life After 3 rd half-life After 4 th half-life Remember: Every time a half-life passes half of a radioactive sample decays (i.e. is reduced by a half!) After 5 th half-life

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Decay curves show the rate of decay for radioactive elements. The curve shows the relationship between half- life and percentage of original substance remaining.

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After 1 st half-life After 2 nd half-life After 3 rd half-life After 4 th half-life Half-life 012345 Time 0573011460171902292028650 Amount (g) Percentage (%) 50.0 100% 25.0 50% 12.5 25% 6.25 12.5% 3.125 6.25% 1.625 3.125% After 5 th half-life

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This type of graph is called an Exponential Decay graph--- it decreases very quickly to start with and approaches zero after a long time (~10 half-lives) Half-lives Mass of Sample (g)

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Notice: It doesnt matter which method you use to plot the data, the shape of the curve is the same! Time (years) Percentage of Sample (%)

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Note: After about 10 half-lives (for Carbon-14 approximately 50000 years) there is so little sample remaining that you cannot measure it accurately enough! Time (years) Mass of Sample (g)

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