Why is radioactive decay measured in half lives?

Question

If you have the # of half lives can you tell how many atoms of that element have decayed?

Answer 1

Because it is the simplest way to solve problems. Radioactive decay is a problem in kinetics, and it is first-order. This means that the rate of the reaction (decay) is proportional only to its own concentration. The more nuclei you are watching, the more likely you are to "see something."

Answer 2

Because each isotope has a characteristic half-life that is independent of the amount of the substance present.