1. (7) Earth in space and time. The student knows that scientific dating methods of fossils and rock sequences are used to construct a chronology of Earth's history expressed in the geologic time scale. The student is expected to: (b) calculate the ages of igneous rocks from Earth and the Moon and meteorites using radiometric dating methods; and (c) understand how multiple dating methods are used to construct the geologic time scale, which represents Earth's approximate 4.6-billion-year history.

2. All matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons (+) in the atomic nucleus. The number of protons plus neutrons (neutral) an element has within its nucleus, is the atomic mass of an element. Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus. What is the atomic number of carbon? 6 What is the atomic mass of carbon? 12 This is an isotope of carbon. What is its atomic mass? number of protons=number of electrons (-) 14

3. 1.Protons have what charge? 2.Electrons have what charge? 3.Neutrons have what charge? 4.How are isotopes different from average elements?

4. Radioactive materials decay at known rates, measured as a unit called half-life. The half-life of a radioactive substance is the amount of time it takes for half of the parent atoms to decay. This is how the material decays over time. The decay of radioactive materials can be shown with a graph, and the parent decays inversely to the daughter evolving. Different isotopes are used to date materials of different ages. Using more than one isotope helps scientists to check the accuracy of the ages that they calculate.

5. Radioactivity provides a way to find the absolute age of a rock. Radioactive dating uses isotopes naturally present in rocks. The radioactive decay of a parent isotope (the original element) leads to the formation of stable daughter isotopes. As time passes, the number of parent isotopes decreases and the number of daughter isotopes increases. You have found a rock and you analyze it. You find it contains 25% parent atoms and 75% daughter atoms. How many half lives have passed? If the half-life of the parent isotope in the sample above is 1 billion years, then how old is the rock? 2 2 billion years old Using the graph, what % of the parent isotope will have evolved into the daughter element after 3 half lives? 87.5% So…radiometric dating is about measuring the RELATIVE amounts of parent isotope, as compared to daughter isotope, present in a sample.

6. 5. What is a half-life? 6. The parent material (isotope) decays _____ to the daughter element as it evolves. 7. How do scientists check the accuracy of the radiometric method they choose to measure the age of a specimen? 8. What is the parent in radiometric dating? 9. What is the daughter in radiometric dating?

7. Radiocarbon dating is used to find the age of once-living materials between 100 and 50,000 years old. This range is especially useful for determining ages of newer human fossils and habitation sites. Carbon isotopes from the charcoal material in these cave paintings places their creation at about 26,000 to 27,000 years BP

8. The Earth’s atmosphere contains three isotopes of carbon: carbon-12, (the most common) carbon-13 and carbon- 14. Only carbon-14 is radioactive; it has a half-life of 5,730 years. The amount of carbon-14 in the atmosphere is tiny and has been relatively stable through time. Plants remove all three isotopes of carbon from the atmosphere during photosynthesis. Animals consume this carbon when they eat plants or other animals that have eaten plants. After the organism’s death, the carbon-14 decays to stable nitrogen-14. The nitrogen atoms are lost to the atmosphere, but the amount of carbon-14 that has decayed can be estimated by measuring the proportion of radioactive carbon-14 to stable carbon-12. As time passes, the amount of carbon-14 decreases relative to the amount of carbon-12. 14 C dating is about measuring relative amounts of that isotope as compared to 12 C. After death, the 14 C is not replaced, but the 12 C remains constant.

9. 10. What are the useful dating boundaries of the 14 C technique? 11. What types of things do scientists date using 14 C. 12. What is the half-life of 14 C, and what is the daughter it decays into? 13. How does 14 C get into plants, and animals?

10. 39 On the periodic table, potassium has an average atomic mass of… Potassium-40 is the radioactive form, and decays to argon- 40 with a half-life of 1.26 billion years. Argon is a gas that escapes from molten magma, so any argon found in an igneous crystal probably formed as a result of the decay of potassium-40. Measuring the ratio of potassium-40 to argon-40 yields a good estimate of the age of that crystal. Potassium is common in many minerals, such as feldspar, mica, and amphibole. With its half-life, the technique is used to date rocks from 100,000 years to over a billion years old. The technique has also been useful for dating fairly young geological materials and deposits containing the bones of human ancestors, as long as they are found in association with volcanic activity.

11. Two uranium isotopes are used for radiometric dating. Uranium-238 decays to lead-206 with a half-life of 4.47 billion years. Uranium-235 decays to form lead-207 with a half-life of 704 million years. Uranium-lead dating is usually performed on zircon crystals. When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but reject atoms of lead. If any lead is found in a zircon crystal, it can be assumed that it was produced from the decay of uranium. Uranium-lead dating is useful for dating igneous rocks from 1 million years to around 4.6 billion years old. Zircon crystals from Australia are 4.4 billion years old, among the oldest rocks on the planet.

12. 14. What does Potassium 40 decay into, and what is its half-life? 15. What minerals commonly contain Potassium 40? 16. What are the two Uranium isotopes we use to measure time, and what are their daughter elements? 17. Where are the oldest rocks on Earth, and where are they?

13. While Radiometric Dating is a very useful tool for measuring precise ages of Earth rocks and organic materials, there are some limitations:  The material being dated must have measurable amounts of the parent and/or the daughter isotopes.  Ideally, for more accuracy, different radiometric techniques should be used to date the same sample; if the calculated ages agree, they can be considered accurate.  Radiometric dating is not very useful for determining the age of sedimentary rocks.  Radiometric dating destroys samples To estimate the age of a sedimentary rock, geologists find nearby igneous rocks that can be dated and use relative dating to determine the age of the sedimentary rock. Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well-defined timeline of Earth history. With information gathered from all over the world, estimates of rock and fossil ages have become increasingly accurate. All of this evidence comes together to pinpoint the age of Earth at 4.6 billion years. Oetzi

14. A scientist is studying a piece of cloth from an ancient burial site. She determines that 40% of the original carbon-14 atoms remain in the cloth. Based on the carbon- decay graph What is the approximate age of the cloth? 8,000 years

15. 18. What are the limitations of radiometric dating? 19. Describe how a scientist might accurately date sedimentary rock formations.

16. Alpha decay is a type of radioactive decay where an alpha particle is emitted. An alpha particle is two protons and two neutrons bound together, which is the same as a helium nucleus. So, an alpha particle is a helium nucleus. So let's zoom into a nucleus and take a look at this alpha decay process. Inside this nucleus, we see the protons and neutrons. This parent nucleus is feeling somewhat unstable because it is too big or simply has too many protons, and it wants to get to a more stable state, so it's going to take two protons and two neutrons and kick them out of the nucleus as we see here. Of course, this process will, over time, turn the isotope into another element.

17. Sometimes radioactive isotopes decay through beta decay. Beta decay is a type of radioactive decay where a beta particle is emitted. A beta particle is an electron that is emitted from the nucleus. What??? Electrons from the nucleus? Essentially, what is happening with beta decay is that we are taking a neutron, and turning it into a proton. Inside this nucleus, we see protons and neutrons, but let's say one of these neutrons is feeling as if things would be more stable if it could turn into a proton. To do that, it emits an electron (the beta particle) and this turns it into a proton.

18. 20. Differentiate between Alpha and Beta decay.