2.  Half-life – the time it takes for ½ of a radioactive sample to decay  Half-life for a radioactive element is a constant rate of decay  Half-life differs for each radioactive substance

4.  For example...strontium – 90 has a half- life of 29 years. If you start with 10 g of strontium – 90 today, how much will remain after 29 years?

5.  Decay curves show rate of decay for radioactive elements  Typically shows the relationship between half-life (y-axis) and percentage of original substance remaining (x-axis).

7.  If we start with 400 atoms of a radioactive substance, how many would remain after one half-life?_________ after two half-lives? _________ after three half- lives? ___________ after four halflives?_________

8. The half-life of Zn-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed?

9. Os-182 has a half-life of 21.5 hours. How many grams of a 10.0 gram sample would have decayed after exactly three half- lives?

10. How much time will be required for a sample of H-3 to lose 75% of its radioactivity? The half-life of tritium is 12.26 years.

11. After 24.0 days, 2.00 milligrams of an original 128.0 milligram sample remain. What is the half-life of the sample?

12.  It can be difficult to determine the ages of objects by sight alone. › Radioactivity provides a method to determine age by measuring relative amounts of remaining radioactive material to stable products formed.

15. Carbon dating measures the ratio of carbon-12 and carbon-14. – Stable carbon-12 and radioactive carbon-14 exist naturally in a constant ratio. – When an organism dies, carbon-14 stops being created and slowly decays. Carbon dating only works for organisms less than 50 000 years old.

16.  Half – Life of Carbon – 14 = 5730 years  Set Up Time vs. % Chart Time (hr) % 0 100 % 5730 50 % 0.195 % 45840 0.39 % 0.78 % 40110 1.56 % 34380 25 % 12.5 % 11460 1719022920 6.25 %3.125% 28650 51570 57300 0.097 %

17.  Write the following Carbon Dating nuclear reactions: › Nitrogen – 14 undergoes neutron capture to become carbon – 14. › Carbon – 14 undergoes beta decay.