Half-Life

HALF-LIFE is the amount of time that it takes for a particular radioactive isotope to decay in half.  Half-life is constant and is not determined by external factors. Usually the more unstable an isotope is the shorter its half-life will be.

Example: Radon-222 has a half-life of roughly 4 days. If there is 20 grams of Radon-222 in a sample, how long will it be until there is only 2.5 grams of it present.

Answer: 12 days. 

This holds true because after one half-life there are only 10 grams of Radon-222 present, after two half-lives there are only 5 grams of Radon-222 present, and after three half-lives there are 2.5 grams of Radon-222 present. Since each half-life is 4 days long and it takes three half-lives for there to only be 2.5 grams of Radon-222 present, the solution is 12 days.

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