[化学] Half-Lives

In discussions of radioactivity, the half-life of anisotope refers to the time it takes for one-half of the sample todecay. If we start with 100 g of a radioactive substance whosehalf-life is 15 days, after 15 days 50 g of the substance will remain.After 30 days, 25 g will remain, and after 45 days, 12.5 g remains, andso on.
    Example

    A radioactive substance has a half-life of20 minutes. If we begin with a 500 g sample, how much of the originalsample remains after two hours?

    Explanation

    The easiest way to attack these questions isto start with the original amount of the sample, then draw arrowsrepresenting each half-life. Two hours is 120 minutes, so that’s sixhalf-lives. At the end of the stated time period, 7.8 g remains.

500 g250 g125 g62.5 g31.25 g15.625 g7.8125 g