7.2: Using Radioactivity to Determine Age

How old is it? How do scientists guess how old things are? How do scientists guess how old things are? One method is to use radioactivity. One method is to use radioactivity. The nuclei of radioactive elements decay at a predictable rate (called a half-life) The nuclei of radioactive elements decay at a predictable rate (called a half-life) Measuring the amount of radioactive material remaining compared to stable decay products lets us guess how long the material may have been decaying (i.e., how old it is). Measuring the amount of radioactive material remaining compared to stable decay products lets us guess how long the material may have been decaying (i.e., how old it is).

Half-life Half-life: time required for ½ of the radioactive nuclei in a sample to decay. Half-life: time required for ½ of the radioactive nuclei in a sample to decay. Each radioactive isotope has its own characteristic half-life, calculated by scientists. Each radioactive isotope has its own characteristic half-life, calculated by scientists. Example: The half-life of Strontium-90 has been calculated to be 29 years. If you have 10 g of strontium-90 today, there will be 5 g remaining in 29 years. Example: The half-life of Strontium-90 has been calculated to be 29 years. If you have 10 g of strontium-90 today, there will be 5 g remaining in 29 years. Strontium g5.0g2.5g1.25gHalf-lives0123Years (29x2) (29x3)

Radioactive Decay Curve Decay curves show the rate of decay for radioactive elements Decay curves show the rate of decay for radioactive elements See pages mass10g5g2.5g1.25g.625g.3125g½-lives012345Years % Strontium-90

Common Isotope Pairs There are many radioisotopes that are used for dating. There are many radioisotopes that are used for dating. Parent isotope = the original, radioactive material. Daughter isotope = the stable product of the radioactive decay.

Radiocarbon Dating for Organic Material Radiocarbon dating measures the ratio of carbon-12 and carbon- 14. It is used in organic material, because it contains carbon. Living things take in carbon from the air and plants. Living things take in carbon from the air and plants. Most the carbon taken in is C-12, but a little bit is radioactive C-14. Most the carbon taken in is C-12, but a little bit is radioactive C-14. As long as an organism lives, it keeps taking in more C-14. As long as an organism lives, it keeps taking in more C-14. When the organism dies, no new C-14 is added and the total amount of C-14 in the organism slowly decreases through radioactive decay. When the organism dies, no new C-14 is added and the total amount of C-14 in the organism slowly decreases through radioactive decay. Scientists guess the age of organic material by measuring the amount of C-14 left. Scientists guess the age of organic material by measuring the amount of C-14 left. Radiocarbon dating can only be applied to organic material less than years old (otherwise there’s too little C-14 to detect) Radiocarbon dating can only be applied to organic material less than years old (otherwise there’s too little C-14 to detect)

Potassium-40 Dating for Rocks Potassium-40 dating measures the ratio of potassium-40 and argon-40 present in rock to guess the age of rock. K-40 decays into Ar-40 with a half-life that scientists have calculated to be 1.3 billion years. K-40 decays into Ar-40 with a half-life that scientists have calculated to be 1.3 billion years. The method assumes that when magma or lava cooled to form the rock, there was no Ar-40, The method assumes that when magma or lava cooled to form the rock, there was no Ar-40, and measures how much exists and measures how much exists now. now.

Problems with Radioactive Dating 1) No one knows the conditions of the radioactive material at its place and time of origin. In carbon dating, what ratio of C-12 and C-14 were in the organism to begin with? In carbon dating, what ratio of C-12 and C-14 were in the organism to begin with? In K-40 dating, was there really no Ar-40 to start with? In K-40 dating, was there really no Ar-40 to start with?

Problems with Radioactive Dating 2) There may have been a gain or loss in parent or daughter materials over time. E.g., Rocks have been found to contain “excess” Ar-40, not formed by radioactive decay but added by Earth’s mantle. In this case, Ar-40 has been added. E.g., Rocks have been found to contain “excess” Ar-40, not formed by radioactive decay but added by Earth’s mantle. In this case, Ar-40 has been added.

Problems with Radioactive Dating 3) It is assumed that the half-life of a radioactive isotope has always been the same value measured today, but this may not be the case E.g., A radioisotope’s rate of decay may change slightly depending on the Earth’s distance from the Sun E.g., A radioisotope’s rate of decay may change slightly depending on the Earth’s distance from the Sun

Problems with Radioactive Dating Radioactive dating is commonly used by scientists today. Radioactive dating is commonly used by scientists today. It is how the age of the Earth has been determined to be ~4.6 billion years old It is how the age of the Earth has been determined to be ~4.6 billion years old It is how this cave painting of horses was determined to be ~30,000 years old It is how this cave painting of horses was determined to be ~30,000 years old However, problems like the 3 described have been used to question the ages determined by radioactive dating methods. However, problems like the 3 described have been used to question the ages determined by radioactive dating methods.

Try These: Practice Problems p.306 #1-4 Practice Problems p.309 #1-3 Homework: Workbook Pages