Half-Life Notes

Half-Life: the amount of time it takes half of a radioactive sample to decay.   Total Time Half-Life # Fraction of Radioactive Atoms Remaining Fraction of Radioactive Atoms Decayed   1 2 3 4 5 6 1 ½ ½ ¼ ¾ 1/8 7/8 1/16 15/16 1/32 31/32 1/64 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 Half-Life # Fraction of Radioactive Atoms Remaining Fraction of Radioactive Atoms Decayed   1 2 3 4 Need 4 half-lives 4 * 1599 years = 6396 years 0 yrs 1 ½ ½ 1599 yrs ¼ ¾ 3198 yrs 1/8 4797 yrs 7/8 1/16 6396 yrs 15/16

Example 2) The half-life of radon-222 is 3. 824 days 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 Half-Life # Fraction of Radioactive Atoms Remaining Fraction of Radioactive Atoms Decayed   1 2 Need 2 half-lives 2 * 3.824 days= 7.648 days 0 days 1 ½ ½ 3.824 days ¼ ¾ 7.648 days

Example 3) The half-life of polonium-218 is 3. 0min Example 3) The half-life of polonium-218 is 3.0min. If you start with 16mg of polonium-218, how much time must pass for only 1.0mg to remain? Starting Amount = 16 mg Remaining Amount = 1.0 mg Fraction Remainig = 1.0 / 16 mg = 1/16 Total Time Half-Life # Fraction of Radioactive Atoms Remaining Fraction of Radioactive Atoms Decayed   1 2 3 4 0 min 1 ½ ½ Need 4 half-lives 4* 3.0min = 12 min 3.0 min ¼ ¾ 6.0 min 1/8 9.0 min 7/8 1/16 12 min 15/16

After 4797 years, how much of an original 0 After 4797 years, how much of an original 0.250g sample of radium-226 remains? Its half-life is 1599 years.

The half-life of radium-224 is 3. 66 days The half-life of radium-224 is 3.66 days. What was the original mass of radium-224 if 0.0800g remains after 7.32 days?