1. Half-Life Nuclear Chemistry

2. What is Half-Life? The time it takes for half of a given amount of a radioactive isotope to undergo decay.

3. Note: The time for the 1 st half life is the same as the time for the 2 nd half life ……. Same isotopes 1/2 1/4

4. Does decay slow down, speed up, or remain constant after each half-life? 1 st Half-Life 2 nd Half-Life Decay slows down If you have to eat ½ in the same amount of time – you have to eat slower. QUESTION? Half-life equal in time

5. What is Half–Life Used For ? Tells the age of a fossil –Carbon-14 Dating 6.25 % of Carbon-14 How many years ago did the volcano erupt?

6. Half-LifeAmount of Carbon% FACT: Carbon – 14 has a half-life of 5730 years 4 x 5730 = 22,920 years 1 2 3 44 1/2 ½ x ½ = 1/4 1/4 x ½ = 1/8 1/8 x 1/2 = 1/16.5 = 50%.25 = 25%.125 = 12.5%. 0625 = 6.25%

7. A rock with 25% of potassium-40 was analyzed. How long ago was the rock formed? Potassium – 40 half-life? 1.25 billion years How many half-life does it take to get 25% of potassium-40? Half-LifeAmount% 1 2 1/2.5 = 50% ½ x ½ = 1/4.25 = 25% 2 2 x 1.25 billion years – 2.5 billions years

8. Ash from a fire pit was found to have 12.5 % as much carbon – 14 as would be found in ash today. How long ago was the ash formed? Carbon-14 half-life? 5730 years How many half-life does it take to get 12.5% of Carbon-14? Half-LifeAmount%.125 = 12.5% 1 2 1/2.5 = 50% ½ x ½ = 1/4.25 = 25% 3 x 5730 years = 17, 190 years 3 1/4 x ½ = 1/83

9. The half-life of radon-22 is 3.8 days. How much of a 100 g sample is left after 15.2 days? Half-LifeAmount Left NumberDays Notice: Total number of days/ half-life days= number of half- lives 1 2 3 4 3.8 7.6 11.4 15,.2 ½ of 100 = 50 g ½ of 50 or ¼ of 100 = 25g ½ of 25 or 1/8 of 100 = 12.5g ½ of 12.5 or 1/16 or 100 = 6.25g 100g

10. Carbon-14 has a half-life of 5,730 years. If a sample contains 70 mg originally, how much is left after 17,190 years? Half-LifeAmount Left NumberDays Mathematical Formula: Total number of days/ half-life days= number of half-lives 17190/5730 = 3 1/8 of original 1/8 x 70 = 8.75 mg 70g 1 2 3 5,730 11,460 17,190 ½ of 70 = 35 mg ½ of 35 or ¼ of 70 = 17.5 mg ½ of 17.5 or 1/8 of 70 = 8.75 mg