1. Half-Life Pg. 870-874

2. Half-Life Half-life = the time needed for half of the radioisotope sample to decay into products Half-live can have varying lengths May be a fraction of a second Can last billions of years Examples: Carbon-14 = 5730 years Potassium-40 = 1.25x10 9 years Radon-22 = 3.8 days Thorium-230 = 75,400 years Thorium-234 = 24.1 days

3. Calculating Half lives There are 2 ways to solve these problems: Continually divide by 2 Use the equation below N = N o (1/2) n

4. Calculating Half-Life Nitrogen-13 emits beta radiation and decays to carbon- 13 with a half-life (t 1/2 ) of 10 min. Assume a starting mass of 2.00g of nitrogen-13. How long is three half-lives? How many grams of the isotope will be present at the end of three half-lives?

5. Calculating with Half lives If the passing of 5 half lives leaves 25.0mg of strontium-90 sample, how much was present in the beginning?

6. Examples of Uses of Half-Life 1.Can be used to determine the age of an artifact (carbon-14 dating) 2.Artificial isotopes tend to have short have lives - Used in medicine - Dont pose long-term hazards to patients asymmetric scan indicates disease normal

7. Radiocarbon dating AKA: Carbon-14 dating Carbon-14 is produced in the atmosphere when high-energy neutrons from space (cosmic rays) collide with nitrogen-14 Carbon-14 decays by - particle production

8. Carbon Dating Carbon-14 is continuously produced in the atmosphere and it continuously decomposes by β- decay The two opposite processes have come into balance, so the amount of carbon-14 present in the atmosphere is approximately constant.

9. Carbon Dating Carbon-14 dating can be used to date wood and cloth artifacts A living plant consumes carbon dioxide in the photosynthesis process Some carbon dioxide contains the C-14 isotope When the plant uses carbon dioxide with C-14, it is incorporated into the molecules in the plant

10. Carbon Dating While it is alive, the C- 14 in the plant molecules is equal to that in the atmosphere. When the tree is cut, there is no more incoming source of C- 14 to balance the C-14 that is decaying. The C-14 content decreases.

11. Carbon Dating Using the known half-life of 5730 years for C-14, archaeologists compare the amount of C-14 in an artifact to the amount currently in trees and figure out the age. A wooden bowl with half as much C-14 as a current living tree would have been through one half life and would therefore be 5730 years old. Carbon dating made by Thomas Athol Rafter

12. Half life Activity You must get the data today Class data will be posted on Moodle tonight Lab due Monday