1. Half-life – length of time it takes for a radioactive substance to decay to half its original mass As a radioactive substance decays, there’ll be less and less of the radioactive substance remaining, while more of the new substance will form. There will come a time during the decay process when exactly half of the original mass of the radioactive substance remains unchanged. The time it takes for the substance to decay to half of its original mass is described as the half-life of the substance. 3 days of decaying Decaying of a radioactive substance is at a constant rate Temperature, pressure, and amount do not change the half-life of a substance Each radioisotope has its own half-life 10 g 5 g new 5 g old

3. Number of half-life period is the number of times a radioactive substance decays in half to go from one mass to another. Original mass Remaining mass 16 g 1 half life> 8 g 2 nd half life> 4 g 3 rd half life> 2 g A.If the original mass and the remaining mass are known in a question: Use arrows to represent one half-life period How many half-life periods does it takes for a radioactive substance to decay from the original mass of 12 grams to the remaining mass of 0.75 g 12  6  3  1.5  0.75 Count the arrows – 4 half-lives

4. B.If the length of time and the half-life are known in a question: Use the equation# of half-life periods = length of time half-life The half-life of a radioisotope is 15 minutes. In 75 minutes, how many half-life periods would the substance have gone through? Knownhalf-life = 15 minutes length of time = 75 minutes # of half-life periods = 75 = 5 15

5. Length of time of decay A.If the half-life and the number of half-life periods are given in a question Length of time = half-life x number of half-life periods Approximately how long will it take for a radioactive 42 K to undergo 6 half-life periods? Knownhalf-life from Table N = 12.4 hours # of half-life periods = 6 Length of time = 12.4 hours x 6 = 74.4 hours

6. B.If half-life, original mass and remaining mass are known How long will it take for K-42 to decay from 100 grams to 12.5 grams? Knownoriginal mass = 100 grams remaining mass = 12.5 grams half-life of K-42 = 12.4 hours 100 g  50 g  25 g  12.5 g3 half-life periods Length of time = 12.5 hours x 3 = 37.2 hours

7. Half-life A.If length of time and number of half-life periods are given in a question Half-life = length of time half-life period What is the half-life of an unknown radioisotope if it takes 12 years for the radioisotope to undergo 5 half-life periods? Knownlength of time = 12 years half-life periods = 5 Half-life = 12 = 2.4 years 5

8. B.If the length of time, original mass and remaining mass are given in a question An unknown substance decays from 64 grams to 2 grams in 1.5 x 10 3 years. What is the half-life of this substance? Knownoriginal mass = 64 grams remaining mass = 2 grams length of time = 1.5 x 10 3 years 64  32 g  16 g  8  4 g  2 g5 half-lives Half-life = 1500 = 300 years 5

9. Determining original mass of a substance A.If the number of half life periods and the remaining mass are given in an equation After 6 half-life periods, 15 grams of an unknown radioisotope remains. What was the original mass of the unknown radioisotope? Knownremaining mass = 15 g # half-life periods = 6 960 g  480 g  240 g  120 g  60 g  30 g  15 g Original mass = 960 g

10. B.If the length of time, half-life and remaining mass are given in a question After 24 days of decaying, a 5 gram sample of 131 I remains unchanged. What amount of the 131 I was present at the beginning of the decay? Knownremaining mass = 5 g length of time = 24 days half-life = 8.07 days # of half-life periods = 24 = 3 8.07 40 g  20 g  10 g  5 g Original mass = 40 g

11. Remaining Mass A.If the number of half life periods and the original mass are given in a question After 4 half-life periods, how many grams of a 12 mg sample f a radioisotope will remain unchanged? Knownoriginal mass = 12 mg # half-life periods = 4 12  6  3  1.5  0.75 mg remain

12. B.If the length of time, half-life and original mass are known in a question In approximately 8 x 10 3 years, how many grams of a 100 gram sample of radon-226 will remain unchanged? Knownoriginal mass = 100 g length of time = 8 x 10 3 years half-life = 1.6 x 10 3 years # of half-life periods = 8000 = 5 1600 100  50  25  12.5  6.25  3.125 g remain

13. Fraction remaining A.If the number of half-life periods of decaying are given in a question Fraction remaining = 1 n = number of half-life periods 2 n What fraction of Au-198 will remain unchanged after 4 half-life periods? Fraction remaining = 1 = 1 2 4 16

14. B.If length of time and half-life of a substance are given in a question Fraction remaining = 1 t = length of time 2 t/T T = half-life What fraction of Ne-19 will remain unchanged after 86 seconds? Knownlength of time = 86 sec half-life = 17.2 sec Fraction remaining = 1 = 1 = 1 2 86/17.2 2 5 32

15. Using fraction remaining and length of time to determine isotope symbol What radioisotope will have 1/16 th of its original mass remaining after about 112 years? Knownfraction remaining = 1/16 length of time = 112 years 1/16 = 1/2 n where n = 4 4 = 112/half-life Half-life = 28 years On table N – Sr-90 has a half-life of 28.1 years.

16. To identify which isotope decays to the greatest extent in a given time period LOOK for the radioisotope with the shortest half-life decays to the least extent in a given time period LOOK for the radioisotope with the longest half-life To determine which radioactive sample has greatest amount remaining after a given period LOOK for radioisotope with LONGEST half-life and MOST mass least amount remaining after a given period LOOK for radioisotope with SHORTEST half-life and LEAST mass

17. Practice Which 10 g sample of these radioisotopes will decay to the greatest extent in 50 years? 1)Kr-852) H-33) Cs-1374) Sr-90 Which sample will have the greatest amount of its isotope remaining after 60 seconds? 1)5 g of Ne-193) 5 g of Fr-220 2)10 g of Ne-194) 10 g of Fr-220

18. The decaying process of a radioisotope can be represented with graphs, diagrams and data tables. Time (days)Mass of radioisotope sample remaining (g) 0160 4120 880 1260 1640 2030 2420 2815 3210

19. Determining mass from the data and graph original mass = mass at time zero = 160 g ½ original mass = mass after 1 half-life period = 80 g1/4 th original mass = mass after 2 half-life periods = 40 g 1/8 th original mass = mass after 3 half-life periods = 20 g Determining time from the data and graph half-life = time when (80 g), ½ original mass remained = 8 days two half-life periods = time when (40 g), ¼ original mass remained = 16 days Three half-life periods = time when (20 g), 1/8 of original mass = 24 days Determining which radioisotope from the data and graph LOOK for a radioisotope on Table N with the half-life of about 8 days = I-131

20. Tracer – radioisotope that is used to follow the path of a chemical reaction Radioisotope tracers in medical treatments and diagnoses must have a short half-life and be quickly eliminated from the body Radioisotope tracers for dating usually have very long half-life

21. Radioisotope name Radioisotope symbol Common application Field of application Iodine-131 131 IThyroid disorder; diagnosis and treatment medical Technetium-99 99 TcCancer tumor diagnosis medical Cobalt-60 60 CoCancer treatment medical Iron-56 56 FeBlood disorder treatment medical Carbon-14 14 CTracer for chemical reaction research Carbon-14 and Carbon-12 14 C 12 C Fossilized datingArcheological dating Uranium-238 and Lead-206 238 U 206 Pb Rock datingGeological dating

22. Radioisotope waste Wastes and radiations produced from nuclear reactors are highly dangerous to life on earth Prolong and high dose exposure to radiation can cause serious health issues, and sometimes death Radiation from nuclear power must be well contained to protect humans and other living things Nuclear wastes are equally dangerous because they are highly radioactive Nuclear wastes need to be stored in safe areas to protect the public from being exposed to them Solid wastes (highly radioactive)Gas wastes 90 Sr 222 Rn 137 Cs 85 Kr 16 N