Rate of Radioactive Decay

How old is this tree? What are some methods you can use to date this tree? Counting rings What if you don’t want to cut it down? What if it’s not a tree?

How old is it? How do scientists find out how old things like these cave paintings of horses are? One method is to use radioactivity. The nuclei of radioactive elements decay at a fixed rate. (This is called a half-life) Measuring the amount of radioactive material remaining compared to stable material lets us calculate how long the material has been decaying. Using radiocarbon dating, these cave paintings of horses, from France, were determined to have been drawn 30 000 years ago. Mesure how radioactivity has changed over time and calculate age of remains All organisms on earth contain carbon, carbon 14 decays when organism dies without being replaced – ratio of carbon 14 to carbon 12 decreases – can tell us how old it is Radiocarbon dating – determine how old something is by measuring amount of carbon 14 remaning

Half-life Half-life: time required for ½ the original nuclei to decay. The half-life for a radioactive element is a constant rate of decay. 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 1 2 3 Years 29 58 87 Strontium-90 10.0g 5.0g 2.5g 1.25g (29x2) (29x3)

Radioactive Decay Curve Decay curves show the rate of decay for radioactive elements Strontium-90 ½-lives 1 2 3 4 5 Years 29 58 87 116 145 % 100 50 25 12.5 6.25 3.125 mass 10g 5g 2.5g 1.25g .625g .3125g Radon-222 has short half life – 3.8 days Uranium-238 has long half life – 4.5 billion years See pages 304-305

Common Isotope Pairs There are many radioisotopes that can be used for dating. This table can be found on p. 307 and on your data booklet. Parent isotope – isotope that undergoes radioactive decay The production of daughter isotope can be a direct reaction or result of a series of decays Parent isotope = the original, radioactive material. Daughter isotope = the stable product of the radioactive decay.

Radiocarbon Dating & the Potassium-40 Clock Radiocarbon dating measures the ratio of C-12 and C-14. When an organism dies, C-14 stops being replenished and slowly decays. Radiocarbon dating only works for organisms less than 50 000 years old. Potassium-40 clock measures the ratio of K-40 and Ar-40. Rutherford created an application for half life radioisotopes when he used constant rate of decay as a clock to help determine the age of Earth At every age of the rock, different amounts of parent and daughter After ONE HALF LIFE, equal amounts of parent & daughter – intersection More daughter – rock is older than one half life

Assignment Classwork: Homework: Practice Problems Q. 1-4 Page 306 Workbook Pages 132-134