Rate of Radioactive Decay

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. (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.

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 See pages 304-305

Common Isotope Pairs There are many radioisotopes that can be used for dating. 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, carbon-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.

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