Page 1 Page 2 Radioactive decay involves the changing of a radioactive atom into another radioactive atom. For example, rocks and minerals may contain potassium-40 that changes into argon-40. Mass spectrometers measure the number of these atoms in a rock or mineral sample. This information and knowing the half-life of potassium-40 can help determine the age of rock layers. The half-life of potassium-40 is 1.3 billion years. The potassium-40 in rocks and minerals will decay through many half-lives until all of them change into argon-40. How many atoms will there most likely be at the third half-life of potassium-40 in this model? A model rock layer had 50 pennies (atoms of potassium-40). How old is the rock layer?