HalfLife Notes HalfLife the amount of time it

Half-Life Notes

Half-Life: the amount of time it takes half of a radioactive sample to decay. Total Time Fraction of Half-Life Radioactive # Atoms Remaining Fraction of Radioactive Atoms Decayed 0 1 0 1 2 ½ ¾ 3 4 5 6 ½ ¼ 1/8 1/16 1/32 1/64 7/8 15/16 31/32 63/64

Example 1) Assuming a half-life of 1599 years, how many years will be needed for the decay of 15/16 of a given amount of radium-226? Total Time Fraction of Half-Life Radioactive # Atoms Remaining Fraction of Radioactive Atoms Decayed 0 yrs 0 1 0 1599 yrs 1 3198 yrs 2 4797 yrs ½ ¾ 3 6396 yrs 4 ½ ¼ 1/8 1/16 7/8 15/16 Need 4 half-lives 4 * 1599 years = 6396 years

Example 2) The half-life of radon-222 is 3. 824 days. How much time must past for ¼ of a given amount of radon to remain? Total Time Fraction of Half-Life Radioactive # Atoms Remaining Fraction of Radioactive Atoms Decayed 0 days 0 1 0 3. 824 days 1 7. 648 days 2 ½ ¼ ½ ¾ Need 2 half-lives 2 * 3. 824 days= 7. 648 days

Example 3) The half-life of polonium-218 is 3. 0 min. If you start with 16 mg of polonium-218, how much time must pass for only 1. 0 mg to remain? Starting Amount = 16 mg Total Time Fraction of Half-Life Radioactive # Atoms Remaining Fraction of Radioactive Atoms Decayed 0 min 0 1 0 3. 0 min 1 6. 0 min 2 9. 0 min ½ ¾ 3 12 min 4 ½ ¼ 1/8 1/16 7/8 15/16 Remaining Amount = 1. 0 mg Fraction Remainig = 1. 0 / 16 mg = 1/16 Need 4 half-lives 4* 3. 0 min = 12 min

After 4797 years, how much of an original 0. 250 g sample of radium-226 remains? Its half-life is 1599 years.

The half-life of radium-224 is 3. 66 days. What was the original mass of radium-224 if 0. 0800 g remains after 7. 32 days?