Absolute Dating Notes and Practice

1. Absolute Dating Notes and Practice

2. Directions:Use the following presentation to complete the notes sheet.

3. Absolute Dating Absolute dating involves methods to determine the age of an event or object in a specific unit of time. To determine the absolute age of fossils and rocks, scientists analyze isotopes of radioactive elements. Isotopes- atoms of the same element with the same number of protons but different numbers of neutrons.

4. Unstable Isotopes Radioactive elements are unstable. They decay, change, into different elements over time. During radioactive decay, the unstableparent isotope decays into stable daughterproduct.

5. Radioactive Decay The half-life of an element is the time it takes for half of the material you started with to decay. Remember, it doesn’t matter how much you start with. After 1 half-life, half of it will have decayed. Each element has its own half-life Each element decays into a new element C14 decays into N14 while U238 decays into Pb206 (lead), etc. The half-life of each element is constant. It’s like a clock keeping perfect time.

6. Over time, your sample will have progressively more daughter material and less parent material. The total amount of matter will not change.

7. Demonstration Time

8. The grid below represents a quantity of C14. Each time you click, one half-life goes by. Try it! C14 – blue (parent) N14 – red (daughter) As we begin notice that no time has gone by and that 100% of the material is C14 Age = 0 half lives (5700 x 0 = 0 yrs)

9. The grid below represents a quantity of C14. Each time you click, one half-life goes by. Try it! C14 – blueN14 - red After 1 half-life (5700 years), 50% of the C14 has decayed into N14. The ratio of C14 to N14 is 1:1. There are equal amounts of the 2 elements. Age = 1 half lives (5730 x 1 = 5730 yrs)

10. The grid below represents a quantity of C14. Each time you click, one half-life goes by. Try it! C14 – blueN14 - red Now 2 half-lives have gone by for a total of 11,400 years. Half of the C14 that was present at the end of half-life #1 has now decayed to N14. Notice the C:N ratio. It will be useful later. Age = 2 half lives (5730 x 2 = 11,460 yrs)

11. The grid below represents a quantity of C14. Each time you click, one half-life goes by. Try it! C14 – blueN14 - red After 3 half-lives (17,100 years) only 12.5% of the original C14 remains. For each half-life period half of the material present decays. And again, notice the ratio, 1:7 Age = 3 half lives (5730 x 3 = 17,190 yrs)

12. C14 – blueN14 - red How can we find the age of a sample without knowing how much C14 was in it to begin with? 1) Send the sample to a lab which will determine the C14 : N14 ratio. 2) Use the ratio to determine how many half lives have gone by since the sample formed. Remember, 1:1 ratio = 1 half life 1:3 ratio = 2 half lives 1:7 ratio = 3 half lives In the example above, the ratio is 1:3. 3) Look up the half life for that isotope and multiply that value times the number of half lives determined by the ratio. If the sample has a ratio of 1:3 that means it is 2 half lives old. If the half life of C14 is 5,730 years then the sample is 2 x 5,730 or 11,460 years old.

13. C14 has a short half life and can only be used on organic (once living) material. To date an ancient rock we use the uranium – lead method (U238 : Pb206). Here is our sample. Remember we have no idea how much U238 was in the rock originally but all we need is the U:Pb ratio in the rock today. This can be obtained by standard laboratory techniques. As you can see the U:Pb ratio is 1:1. From what we saw earlier a 1:1 ratio means that 1 half life has passed. Rock Sample Now all we have to do is see what the half-life for U238 is. 1 half-life = 4.5 x 109 years (4.5 billion), so the rock is 4.5 billion years old. Try the next one on your own.............or to review the previous frames click here.

14. Element X (Blue) decays into Element Y (red) The half life of element X is 2000 years. How old is our sample? See if this helps: 1 HL = 1:1 ratio 2 HL = 1:3 3 HL = 1:7 4 HL = 1:15 If you said that the sample was 8,000 years old, you understand radioactive dating. If you’re unsure and want an explanation just click.

15. Element X (blue) Element Y (red) How old is our sample? We know that the sample was originally 100% element X. There are three questions: First: What is the X:Yratio now? Second: How many half-lives had to go by to reach this ratio? Third: How many years does this number of half-lives represent? 1) There is 1 blue square and 15 red squares. Count them. This is a 1:15 ratio. 2) As seen in the list on the previous slide, 4 half-lives must go by in order to reach a 1:15 ratio. 3) Since the half life of element X is 2,000 years, four half-lives would be 4 x 2,000 or 8,000 years. This is the age of the sample.

16. Try it: "What is the half-life of this element?" Just remember that at the end of one half-life, 50% of the element will remain. Find 50% on the vertical axis, Follow the blue line over to the red curve and drop straight down to find the answer: The half-life of this element is 1 million years.

17. Try It: "What percent of the material originally present will remain after 2 million years?" Find 2 million years on the bottom, horizontal axis. Then follow the green line up to the red curve. Go to the left and find the answer. After 2 million years 25% of the original material will remain.

18. How to Solve Half Life Problems • Read over the steps for how to solve half life problems on the front of the WS. • Use these process to solve the questions on the back of the worksheet. • When you are done, check your answers on the last slide of this presentation.

19. Key to Half Life Problems • 0.125g • 25 mg • 0.625 g • 6 years • 75 minutes • 0.0625 g would remain after 15 minutes. You would need to order about 100 g.

20. Directions Go to http://www.glencoe.com/sites/common_assets/science/virtual_labs/E18/E18.html Follow the directions on the left side of the screen to simulate radioactive decay.