Radioactivity provides a method to determine the age of a material

Carbon is use to determine age of a material most living organisms are based on organic molecules (i.e. contain carbon). When organisms die carbon-14 stops being created and slowly decays C-14 has a Half Life of 5730 years - the time it takes for a radioactive isotope to be halved is called Half-life So by measuring the amount of carbon-14 remaining in a sample and then comparing it to the amount of carbon-12 in the sample we can determine the age of the material Measuring the relative amounts of carbon-12 : carbon-14 is called radiocarbon dating

The grid below represents a quantity of C14. Each click is one half-life (5730 years) . C14 – blue N14 – red Half lives % C14 %N14 Ratio of C14 to N14 100% 0% no ratio 1 50% 1:1 2 25% 75% 1:3 3 12.5% 87.5% 1:7 After 3 half-lives (17,100 years) only 12.5% of the original C14 remains. For each half-life period half of the material present decays. And again, notice the ratio, 1:7 Age = 3 half lives (5730 x 3 = 17,190 yrs)

Table of radioisotopes: 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.

Decay of a 50.0 g radioactive sample of C-14 After 2nd half-life After 1st half-life 12.5 g After 5th half-life After 4th half-life 6.25 g 1.625 g 3.125 g After 3rd half-life Half-life 1 2 3 4 5 Time 5730 11460 17190 22920 28650 Amount (g) Percentage (%) 50.0 100% 25.0 50% 12.5 6.25 3.125 1.625 25% 12.5% 6.25% 3.125%

Decay curves The curve shows the relationship between half- life and percentage of original substance remaining.

The Potassium-40 Clock Radioisotopes with very long half-lives can help determine the age of very old things. Potassium-40 is a key element. The potassium-40 to argon-40 has a half-life of 1.3 billion years. Potassium-40 can exist in hot molten rock, while argon-40, the daughter isotope, escapes from the molten rock because it is a gas. Argon-40 produced by the decay of potassium-40 becomes trapped in rock. Ratio of potassium-40 : argon-40 shows age of rock.