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Published byGenevieve Moke Modified over 2 years ago

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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 years old (… more on why later!) Using radiocarbon dating, these cave paintings of horses, from France, were determined to have been drawn 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 Time Amount (g) Percentage (%) % % % % % % 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 years) there is so little sample remaining that you cannot measure it accurately enough! Time (years) Mass of Sample (g)

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