Radioactive M&Ms
1. If you started with a sample of 600 radioactive m&mÕs, how many would remain undecayed after three half-lives?  
2. If 175 undecayed m&mÕs remained from a sample of 2800, how much time has passed? 
3. How many half-lives would it take for 6.02 x 1023 nuclei to decay to 6.25% (0.376 x 1023) of the original number of nuclei? 
4. Is there any way to predict when a specific piece of candy will land marked side up or Òdecayed?Ó If you could follow the fate of an individual atom in a sample of radioactive material, could you predict when it would decay?  Explain. 
5. Strontium-90 has a half-life of 28.8 years. If you start with a 10-gram sample of strontium-90, how much will be left after 115.2 years? Justify your answer. 
6. The half life of the radioactive isotope iodine-131 is 8 days. Calculate the amount of iodine-131 that would be left in a 100 gram sample after 8, 16, 32 and 80 days. 
7. You are given 10 grams of pure uranium-238 (U) on January 1.  Its half-life is 4? billion years, and it decays to form thorium-234 (234Th). 
a) How long would you have to wait before half of it turned into thorium-234? 
b) How much would you have left after 9 billion years? 
c) How long would you have to wait before you had only .5 grams left?