Absolute Dating Notes and Practice Directions Use the

Absolute Dating Notes and Practice

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

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.

Unstable Isotopes Radioactive elements are unstable. They decay, change, into different elements over time. During radioactive decay, the unstable parent isotope decays into stabl daughter product.

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 C 14 decays into N 14 while U 238 decays into Pb 206 (lead), etc. The half-life of each element is constant. It’s like a clock keeping perfect time.

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

Demonstration Time

The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! of C 14 – blue (parent) N 14 – red (daughter) Half % C 14 %N 14 CRatio to N 14 lives 0 100% 0% no ratio As we begin notice that no time has gone by and that 100% of the material is C 14 Age = 0 half lives (5700 x 0 = 0 yrs) 14

The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! Ratio of Half % C 14 %N 14 14 14 C to N C – blue N - red lives 14 Age = 1 half lives (5730 x 1 = 5730 yrs) 14 0 100% 0% no ratio 1 50% 1: 1 After 1 half-life (5700 years), 50% of the C 14 has decayed into N 14. The ratio of C 14 to N 14 is 1: 1. There are equal amounts of the 2 elements.

The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! Ratio of Half % C 14 %N 14 14 14 C to N C – blue N - red lives 14 Age = 2 half lives (5730 x 2 = 11, 460 yrs) 14 0 100% 0% no ratio 1 50% 1: 1 2 25% 75% 1: 3 Now 2 half-lives have gone by for a total of 11, 400 years. Half of the C 14 that was present at the end of half-life #1 has now decayed to N 14. Notice the C: N ratio. It will be useful later.

The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! Ratio of Half % C 14 %N 14 14 14 C to N C – blue N - red lives 14 Age = 3 half lives (5730 x 3 = 17, 190 yrs) 14 0 100% 0% no ratio 1 50% 1: 1 2 25% 75% 1: 3 3 12. 5% 87. 5% 1: 7 After 3 half-lives (17, 100 years) only 12. 5% of the original C 14 remains. For each half-life period half of the material present decays. And again, notice the ratio, 1: 7

C 14 – blue N 14 - red In the example above, the ratio is 1: 3. How can we find the age of a sample without knowing how much C 14 was in it to begin with? 1) Send the sample to a lab which will determine the C 14 : N 14 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 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 C 14 is 5, 730 years then the sample is 2 x 5, 730 or 11, 460 years old.

C 14 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 (U 238 : Pb 206). Here is our sample. Remember we have no idea how much U 238 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. Rock Sample 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. Now all we have to do is see what the half-life for U 238 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.

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.

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: Y ratio 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.

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.

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.

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.

Key to Half Life Problems 1. 2. 3. 4. 5. 6. 0. 125 g 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.

Directions Go to http: //www. glencoe. com/sites/common_ass ets/science/virtual_labs/E 18. html Follow the directions on the left side of the screen to simulate radioactive decay.