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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.
radioactive decay
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!