Half-Life
For the SAT Chemistry Subject Test, you should know everything we just said
about radioactive decay, and you should also know about the rate of radioactive
decay. The rate of radioactive decay of a substance is called its half-life. For
example, if we start with 1,000 g of a radioactive substance, and its half-life is 1
year, then after 1 year we’ll have 500 g of the original sample left. After another
year we’ll have 250 g of the original sample left, and so on. That’s how half-
lives work. Let’s take a look at the rate of decay, illustrated on the following
page.