Part 4

The planet--or center of the atom--is called the nucleus. It is made mostly of protons and neutrons. The little satellites circling around the nucleus are always electrons.

Ordinary atoms always have the same number of protons as they do of electrons. This number is called the atomic number. No two elements have the same atomic number. It is the number of protons and electrons that tells us what kind of atom it is. When there are two protons in the nucleus and two electrons circling around it, we know that we have an atom of helium. If it doesn't have two protons and two electrons, it isn't helium.

A helium atom also has two neutrons in its nucleus. Usually, no atom can change its number of protons or electrons and remain the same kind of atom--but it can change its number of neutrons.

Atoms of the same element, but with different numbers of neutrons in the nucleus, are called isotopes. Some elements have only one isotope, some have as many as eight or ten.

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!

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.

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!

Radioactivity is very interesting, but before we can understand its real importance, we must learn a little about energy.

To most of us, energy means "pep." To a scientist, energy means the ability to do work. Energy is not a "thing." You can't see it. You can only see--or hear--or feel--what it does. Energy never disappears, but it can be changed from one form to another.

When you swing a bat and wallop a ball, part of the energy you use makes the ball whiz through the air. If you use energy to clap your hands, part of the energy is changed to sound, and you hear a noise. If electrical energy is used in a light bulb, part of the energy is changed into light and part into heat.

When we burn wood for heat, we are using energy that the tree took from the sun. When we burn coal or oil, we are using the energy of sunlight that was stored many millions of years ago.

All of this energy is stored in the atoms of the wood, coal, or oil. But when we burn these materials for fuel, we release only the energy of the electrons.

Now do you remember, back in the last chapter, we said that the nucleus is the heavy part of the atom? And that the electrons are very light? Well, the nucleus is so very, very heavy for its tiny size, that it cannot be compared to anything else in the world. If a nucleus were as large as a grain of rice, it would weigh two million tons! Nothing so small could weigh so much unless it were extremely tightly packed together. It takes a great deal of energy to pack anything that solidly.

By the middle of the 1930's, scientists were beginning to think about the huge amount of energy that would be released if the nucleus could be split. The scientific name for splitting is fission.

Just suppose, the scientists thought, you could split a nucleus and its neutrons would come flying out--and each neutron would strike like a bullet at another nucleus and make that one split? And all the new flying neutrons would split other atoms? This would be a chain reaction.

If man could produce a chain reaction, there would be such energy as the world never dreamed of! In many different countries, men thought, and dreamed, and worked--the search for the nuclear chain reaction was on!

JOURNEY TO THE NEW WORLD

It was a gray winter morning. The date was December 2, 1942. The place, The University of Chicago. Here at Stagg Field, under the football stands, was a large empty room that had once been a squash court.

None of the students who hurried by on the way to class paid much attention to a few men who passed through the door into the long unused room. No one knew that in that room one of the greatest events in the history of science was about to take place. No one knew that the atomic age would be born that day.