Uranium has three main isotopes. The most common kind of uranium has 92 protons, 146 neutrons, and--of course--92 electrons. (Did you remember that the number of electrons has to match the number of protons?) Adding up the total particles in the nucleus, we see that 92 + 146 = 238, so this kind of uranium is called uranium-238. There is an isotope that has 143 neutrons, so this is uranium-235 because 92 + 143 = 235. The last isotope has 142 neutrons and is uranium-234. The heaviest part of any atom is the nucleus. Protons and neutrons are very much heavier than electrons. And then there is lots and lots of empty space. If the nucleus of the hydrogen atom (which has only one proton) were enlarged to the size of a tennis ball, the electron would be a half mile away! A whole atom is so tiny that it is almost impossible to imagine anything so small. It would take 250 million of some kinds of atoms to measure one inch. But it would take _fifty thousand_ times as many electrons to cover the same inch! [Illustration] 5 A SEARCH BEGINS Now we know a lot about the structure of the atom--but we still haven't solved the mystery of the rays. So let's do that right now. A radioactive atom is really a temporary atom. It is unbalanced and shoots off parts of itself in order to become balanced. As the atom gives up protons, neutrons, and electrons, we say that it _decays_. If your tooth decays, a small part of it crumbles away. And the same thing happens to an atom. We know that if an atom changes its number of protons it becomes a different kind of atom. And that is just what happens to radioactive elements. Uranium, thorium, and radium all change into lead. Other radioactive elements decay to different elements. When an atom decays, it gives off three different kinds of particles. These are named for the first three letters of the Greek alphabet and are called _alpha_ particles, _beta_ particles, and _gamma_ rays. [Illustration] Some radioactive elements decay very quickly--in a few seconds--but some take millions of years. As the element decays, its atoms shoot off particles. The larger the amount of the element, the more particles it shoots off. But as the element decays, there is less and less of it left. If at first it gives up 100 particles a second, it will, as its size decreases, give up only 90 particles a second. Then it will give up only 80 particles a second, and so on. This slowing down makes it very hard to measure how long it will take the element to completely decay. It is much easier to figure out when it will be _half_ decayed. And so we never speak of the life of a radioactive element. We speak of its _half-life_. When the radioactive element that started giving up 100 particles a second gets down to losing only 50 particles a second, we know that half of its radioactivity has been used up. Radium has a half-life of 1,690 years. Uranium has a half-life of 4,500 million years!