Half-Life and Half-Life Calculations

Half-Life and Half-Life Calculations Name______Period _____ Date ______Half-life . When a radioactive isotope changes into another element, we say it has decayed. . Each isotope of an element has a constant and characteristic rate of “decay.” . The rate of decay is measured in half-lives. . A half-life is the time it takes for half of the radioactive isotope to decay. . Knowing a radioactive isotope's half-life makes it possible to estimate the age of a sample.

Solving half-life problems. In half-life problems several pieces of information are given and you are asked to find another. Setting up the following table will help keep the different types of information straight. You may not be given, nor will you need, information for all columns.

# Half lives Time % Left Amount left 0 hl 0 unit 100% ____ unit

Add all values given in the problem and work through the table until you get the answer requested. Keep track of units! Remember for each half-life, the % and the Amount left is reduced by half.

Sample problems: A. If the original amount of radon-222 was 500 grams, and 62.5 grams are left, how many half-lives have passed?

# Half lives Time % Left Amount left 0 hl 0 days 100% 500 grams 1 -- 50 250 2 -- 25 125 3 -- 12.5 62.5 g

Answer: It took 3 half-lives for 500 g of radon-222 to be reduced to 62.5 g.

B. The half-life of radon-222 is 3.8 days. How many half lives have passed in 15.2 days?

# Half lives Time % Left Amount left 0 hl 0 days 100% -- 1 3.8 50 -- 2 7.6 25 -- 3 11.4 12.5 -- 4 15.2 6.25 --

Answer: 4 half-lives have passed in 15.2 days.