 In chapter 3, you learned about protons, neutrons, and electrons. You learned about the basic structure of the atom, the structure of the nucleus, and isotopes. Most of the atoms around you are stable and have been stable for a very long period of time. There are, however, some atoms that have an unstable ratio of the number of neutrons to protons. They undergo decay and emit particles. We call this radiation. We have a Geiger counter that measures radioactivity. You can see that we get, every once in a while, reading on our dial, which is the background radiation that we're exposed to all the time. I have a watch that I had as a child, which had numbers that glowed in the dark. The numbers on the watch were painted with a fluorescent paint mixed with a very small amount of radium, element number 88. When the radium decays, it produces alpha particles and gamma rays. The alpha particles are stopped by the glass in the watch. The gamma rays come through. If I bring the watch to the Geiger counter, I can measure the radiation.