1. Radiometric Dating “clocks in rocks”

2. Absolute Dating Gives a numerical age Gives a numerical age Works best with igneous rocks & fossils Works best with igneous rocks & fossils Uses isotopes Uses isotopes

3. Isotopes- different number of neutrons Carbon 14 ( 14 C) Carbon 14 ( 14 C) –2 “extra” neutrons –radioactive

4. Common dating isotopes ParentDaughter Half life Potassium 40 Argon 40 1.3 by Rubidium 87 Strontium 87 48.8 by Uranium 235 Lead 207.7 by Carbon 14 Nitrogen 14 6,000 years Sodium 22 Aluminum 27 15 hours

5. radioactivity Nuclei break apart Nuclei break apart Emit particles or waves (radiation) Emit particles or waves (radiation)

6. Ionizing Radiation Radiation Alpha Particle Neutron Particle Beta Particle Radioactive Atom Gamma Ray (X Ray)

7. Average Annual Dose Cosmic 28 mrem Terrestrial 28 mrem Internal 40 mrem Medical X-Rays 40 mrem Nuclear Medicine 14 mrem Consumer Products 10 mrem Other 3 mrem Radon 200 mrem Natural Sources Man-Made Sources

8. Comparison of Radiation Dose Doses shown in bar graph are in units of mrem

9. Half-Life 0 200 400 600 800 1000 1200 New1 Half- Life 2 Half- Lives 3 Half- Lives 4 Half- Lives Activity The time required for the amount of radioactive material to decrease by one-half

12. Half-lives ParentDaughter Half life Potassium 40 Argon 40 1.3 by Rubidium 87 Strontium 87 48.8 by Uranium 235 Lead 207.7 by Carbon 14 Nitrogen 14 6,000 years Sodium 22 Aluminum 27 15 hours

13. Half life problem If you begin with 80 grams of 14 C after a time, 20 grams are left. How old is the sample? If you begin with 80 grams of 14 C after a time, 20 grams are left. How old is the sample?

14. 14 Carbon Only accurate for ages less than 100,000 years Only accurate for ages less than 100,000 years Parent gets too small to accurately measure Parent gets too small to accurately measure

15. Mass spectrometer measures D/P ratio

16. Preparing a rock for mass spectrometer

17. Using daughter/parent ratio 1) D/P ratio 1) D/P ratio 2) figure out number of half-lives 2) figure out number of half-lives –Use graph 3) multiply number of half-lives by the time of one half-live 3) multiply number of half-lives by the time of one half-live Example: 240 g 14 N ; 8 g 14 C Example: 240 g 14 N ; 8 g 14 C

18. Another half-life problem 1,000 grams of radioactive element is in a rock when it is formed. The element’s half-life is 2 million years. After a time, 125 grams of the original element remain. How old is the rock? 1,000 grams of radioactive element is in a rock when it is formed. The element’s half-life is 2 million years. After a time, 125 grams of the original element remain. How old is the rock?