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1. The graph below shows the decay of a radioactive isotope. What is the halflife ofthe isotope? Radioactive atoms 30 - - - 0 50 100 150 200 A. 15 seconds B. 25 seconds C. 40 seconds D. 200 seconds 2. Which of the following is the best denition of halflife for a radioactive substance? A. Half of the amount oftime required for all ofthe radioactive atoms to decay. B. "he amount of time required for half of the radioactive atoms to decay. C. "he amount of time required for each radioactive atom to decay halfway. D. All ofthe above. 3. Suppose 128 radioactive atoms have a halflife of 15 seconds. Approximately how many radioactive atoms will be left after 60 seconds? A. 32 B. 16 C. 8 D. None 4. Two scientists are experimenting with pure samples of isotope X, a highly radioactive substance. The rst scientist has a sample with a mass of 20 grams. He measures a half-life of 232 seconds. The second scientist has a sample ofthe same substance with a mass of 80 grams. What is the halflife that she is most likely to measure? A. 58 seconds B. 232 seconds C. 464 seconds D. 928 seconds 5. Radioactive isotopes can be used to nd the age of rocks, fossils, or other artifacts. Carbon1 4 has a halflife of 5,730 years. Suppose a sample of charcoal from a primitive re pit contains one eighth of its original amount ofcarbon-14. How old is the sample? A. 716 years B. 17,190 years C. 22,920 years D. 45,840 years