absolute dating notes

1. Absolute Dating Notes

2. Absolute dating of Fossils and Rock Layers with Radioisotopes (In order to understand radioisotope dating of fossils and rocks, let’s review a few basics in chemistry of elements:)

3. Structure of an atom: Atoms have three basic types of particles: Protons Neutrons electrons

4. Protons Particles in the center or nucleus of the atom that have a + charge.

5. Neutrons Particles located in the nucleus that have no charge.

6. Electrons Very tiny particles orbiting the nucleus that carry a negative (-) charge.

7. What is an Atomic Number? The number of protons contained in the nucleus of the atom.

8. What is the atomic mass? It is close to the number of protons plus the number of neutrons.

9. What makes an atom neutral in charge? When the number of electrons matches the number of protons.

10. What is the atomic stucture of a regular carbon atom? 6 P+ 6 N 6 e-

11. What is the structure of radioactive carbon- 14? 6 P+ 8 N 6 e- (2 extra neutrons)

12. What is an isotope? Atoms with the same number of protons, but different numbers of neutrons. So Carbon 14 is an isotope of regular carbon 12.

13. Carbon-14 Is unstable and wants to change into a regular Nitrogen-14 atom by changing one of its extra neutrons into a proton and an electron. 7 P+ 7 N = stable Nitrogen 7 e-

14. What is a Beta Particle? The electron given off by an extra neutron to change it to a proton: N = P+ plus e- 6 P+ ? 7 P+ 8 N ? 7 N 6 e- ? 7 e-

15. Why does unstable C-14 occur? Nitrogen in the upper atmosphere is bombarded by gamma rays from the sun.

16. Creation of Carbon 14

17. What is the ratio of C-14 to C-12 in Earth’s atmosphere? 1 C-14: 1,000,000,000,000 C-12 This ratio is fairly constant in our atmosphere.

19. How does the C-14 get into fossils? Plants absorb C-14 and C-12 during photosynthesis in the form of _______.

21. Animals eat plants Now their bodies contain the same known ratio of C-12 and C-14.

22. As long as an organism lives, it replaces the decaying C-14 and the C-12 in its cells.

23. What happens when the animal or plant dies? No new C-14 is absorbed, and the ratio starts going down.

25. Radioactive decay Causes the C-14 to slowly disappear as it changes to Nitrogen. N = P+ and e-

26. What is a half-life of an isotope? It is the time it takes for one half of the molecules in a sample to decay to the stable isotope.

27. When the quantities of parent and daughter atoms are equal (ratio 1:1), how many half-lives have transpired? (paper and scissors demo)

28. What is the actual half-life of C-14? 5,730 years

29. Carbon 14 Can be used to date things up to about 7 half-lives. (5,730 x 7) ˜ 40,000 years

31. How can we date things older than 40, 000 years or non-living things like rocks? The isotope Uranium 238 is often used. U-238 ? Thorium 230 + Helium ? Lead 206

32. What is the half-life of U-238? 4.5 billion years

33. A third system of isotopes used for absolute dating Potassium-40 which decays to its stable daughter product Argon-40. This system can accurately date rocks that are less than 2 million years old. (It is useful because many rocks contain potassium.)

34. When does the U-238 or K-40 clock start ticking? (since they aren’t living and so don’t die) When igneous or metamorphic rocks cool and form their crystal structures.

36. Can sedimentary rock be dated this way, using isotopes? No, scientists must relate sedimentary layers to datable igneous rock layers and use relative dating laws to determine their ages. Law of Superposition Law of Crosscutting relationships

38. Fossils Can help with the dating of sedimentary rock layers.

39. Summary Using relative dating and absolute dating, paleontologists can tell how old the Earth and its ancient life are!

40. The End! (Whew!)