The peaceful atom — Reading Companion

Dalton said that all elements are made of atoms, and the atoms of any particular element are always the same. An atom of carbon is always like every other atom of carbon. And, Dalton went on, atoms of different elements have different weights. An atom of carbon weighs more than an atom of hydrogen. An atom of oxygen weighs more than an atom of carbon.

Dalton noticed that atoms combined in different ways according to their weights. Water is made of hydrogen and oxygen. But it takes _two_ hydrogen atoms with _one_ oxygen atom to make water. This smallest unit of water is called a _molecule_. A molecule is the smallest possible amount of any chemical compound.

Another contribution that Dalton made was the use of shorthand symbols to stand for chemical elements. Today, we use a system which grew out of Dalton's.

Instead of calling the water molecule "two atoms of hydrogen and one atom of oxygen," we call hydrogen "H" and oxygen "O" and write, simply, H_{2}O. The symbol for carbon is "C" and carbon dioxide may be written CO_{2}. This means that one carbon atom and two oxygen atoms combine to form one molecule of carbon dioxide.

John Dalton was a great scientist and almost everything in his atomic theory turned out to be correct. Only one of his ideas we now know to be wrong. And that idea went all the way back to Democritus. They both thought that the atom was the smallest possible particle, and that it could never be divided.

It certainly seemed to be so. But astonishing things about atoms began to turn up around the end of the 1800's. No one knew it yet, but men were going to change atoms--and atoms were going to change the world!

THE MYSTERY OF THE RAYS

In 1895, a German scientist, Wilhelm K. Roentgen (RENT-gen), was experimenting with a special kind of electrical tube. He had covered one end of the tube with black paper. Nearby was a screen that glowed when light shone on it.

Roentgen happened to glance at the screen--and could hardly believe his eyes. It was glowing--but there was no light coining from the covered tube! This seemed impossible--but there it was.

Later, when someone asked Roentgen what he thought, he made a true scientist's reply.

He said, "I did not think. I investigated!"

Roentgen didn't know what the rays were, so he decided to call them _X rays_. He found that they could pass not only through black paper, but through many other substances as well. They went easily through cloth or wood, but were slopped by metal. Roentgen found that the mysterious rays could shine right through the soft parts of the body, but they were stopped by bones.

Here was a useful discovery, indeed. Just imagine how valuable it was to a doctor to be able to take a shadow picture of a broken bone so that he could see how to set it. Your dentist probably uses Roentgen's X rays to take pictures of your teeth to look for cavities.

Roentgen's discovery stirred up a great deal of interest. Perhaps there were other kinds of rays. Scientists began to search for them.

One of the searchers was Antoine Henri Becquerel (Beck-er-EL), a Frenchman. In 1896, Becquerel was experimenting with some crystals of a uranium salt. These crystals gave off a glow after being exposed to sunlight.

One day, Becquerel was all ready to test the glow on a photographic film. But just as he was about to start, the sky turned cloudy. The experiment couldn't be done without sunlight, so Becquerel wrapped his film in a piece of black paper, put the crystal on top, and put the package away in a drawer.

The sun stayed hidden for several days and Becquerel couldn't continue with his experiment. But he decided to develop his film anyway. He was astonished to find a black spot right in the middle, just where the uranium crystal had been! That meant that even without any sunlight, the crystal had given off rays of its own! Another mystery!

Becquerel tested several compounds that contained uranium and found that they all gave off similar rays. Why?

One of the scientists who heard about Becquerel's work was a young Polish woman working in Paris. Her name was Marie Curie (Cu-REE).

Madame Curie wondered if uranium were the only element to give off rays. With the help of her husband, Pierre, she began to test every known element for rays. She decided to call this ability to give off rays _radioactivity_.

After many tests, Madame Curie found that the element _thorium_ was also radioactive. So, the Curies reasoned, if there were _two_ radioactive elements, there were probably more. And they continued to search.

Then one day, a strange thing happened. The Curies were busy refining uranium from its ore, _pitchblende_. But they suddenly noticed that the ore seemed to be more radioactive than the uranium itself! How could this be? The only explanation, Madame Curie thought, was that there must be another, stronger, radioactive element in pitchblende.

In 1898, after working through tons of pitchblende, the Curies succeeded in separating a speck of a new element that was 900 times more radioactive than uranium! They named the new element _radium_.

The Curies and other scientists were very excited. Here were three elements--uranium, thorium, and radium--all giving off powerful rays. Where did the rays come from? Scientists were sure they could only come from the atoms of the elements themselves. But how could that be? There couldn't be anything smaller than an atom. Or could there? The scientists didn't know. It was a real mystery.

Every mystery is sure to attract some people who wish to solve it. And the mystery of radioactivity was no exception. Scientists in many parts of the world began to search for clues. Little by little, they found them. It wasn't until the 1930's that the last pieces of the puzzle began to fall into place--the structure of the atom was finally clear.

But before we talk about the structure of atoms, let's talk about the structure of houses. Many houses are built of wood, shingles, and glass. However, even though the houses are built of the same materials, they may not look anything alike. Some are ranch houses, some are split levels, and some are colonial houses.

On the other hand, sometimes a builder puts up a large development in which all the houses are exactly the same. If a new friend tells you that he lives in the Shady Acres development, you can picture his house. It is just like every other house in Shady Acres.

Are you wondering what this has to do with atoms? Well, all atoms are built of the same principal materials.

They are called _protons_, _electrons_, and _neutrons_. And just as a ranch house never looks exactly like a colonial house, an atom of one element never looks exactly like the atom of another element.

But like the Shady Acres houses, every atom of the same element looks exactly like every other. A hydrogen atom looks like every other hydrogen atom. A carbon atom looks like every other carbon atom. But a hydrogen atom never looks like a carbon atom.

Atoms are like houses in still another way, too. Even though the building materials used in two houses or two atoms are the same, the finished structure of a house or an atom depends on the way the materials are arranged.

If an atom could be made large enough for you to see, you might think you were watching a satellite (or a fleet of satellites) going around and around one or more planets.

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!

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.

The men who had gathered in the secret room were some of the finest scientists in the world. The leader of the group was Enrico Fermi (En-REE-ko FER-mee), an Italian scientist who had come to the United States.

For some weeks the men had been quietly at work, carefully stacking a huge pile of pure graphite bricks. Here and there among the bricks they placed pieces of uranium. Fermi believed that when the pile reached a certain size, a chain reaction would start. By December 2, the size seemed to be right.

Inside the pile were three control rods. They were made of cadmium, an element which soaks up flying neutrons like a sponge. With the rods in place, no reaction could take place. When the rods were withdrawn, the reaction would begin.

To make sure that the pile would not get out of hand, the three control rods were operated in three different ways. The first one was controlled by an electrical switch and was completely automatic. The second, called ZIP, was tied to a rope in the balcony. In case of emergency, there was a man ready with an axe. He had only to chop the rope and ZIP would go crashing back into the pile. The third rod was moved by hand.

It was time to begin. Fermi gave the signal for the automatic rod to be withdrawn. Immediately, the counters which measured radioactivity began to tick.

Then Fermi gave the command, "ZIP out!" ZIP was drawn up on its balcony rope and the ticking of the counters at once became faster.

Then Fermi turned to the man who controlled the last rod. This rod was marked in feet and inches, and Fermi said: "Pull it out to thirteen feet."

All eyes were on the instruments. Not yet. A little more. Pull it out another foot. Not yet. The men grew more and more tense as the careful work went on.

Finally, at about 3:25 in the afternoon, Fermi made a last check of his instruments and his calculations. Then he said: "Pull it out another foot. This is going to do it!"

No one dared breathe. The ticks of the counters became so rapid they sounded like a steady hum. The pointers on the instruments swung all the way over--and stayed there. The first atomic chain reaction had been achieved!

The pile was allowed to run for 28 minutes. Then the control rods were put back. Suddenly, all was quiet. There were no ticks from the counters.

Not only had these men started a chain reaction, they had also been able to stop it. At last man could control the energy of the atom.

One of the men present, Arthur H. Compton, ran to the phone to call James B. Conant, chairman of the U. S. National Defense Research Committee. But since our country was at war in 1942, it wasn't safe to talk about this important secret over the telephone. And so, on the spur of the moment, a quick-witted and historic conversation took place.

Compton said: "Jim, you'll be interested to know that the Italian navigator has just landed in the new world."

Conant, who knew of the experiments that had been going on, understood at once. He said: "Is that so? Were the natives friendly?"

And Compton replied: "Everyone landed safe and happy."

This was the first day of the atomic age. The reactor had been started, had been stopped--and had produced enough power to light one small flashlight bulb!

TINY ATOMS, BIG POWER

Atomic power has grown quickly since that day in 1942. Atomic power plants now make electricity to light large cities in many parts of the world.

Atomic power doesn't make electricity directly. It makes heat. The heat turns into steam. Then the steam turns turbines and the spinning turbines drive the generators which make electric current.

Ordinary steam power plants depend on fossil fuels--coal, oil, or gas--to make heat. It has been figured out that if only coal were used for fuel, the world's supply would be used up in 350 years. Oil and gas would last for 40 years. But there are enough nuclear fuels to last for at least 8,500 years!

There are several kinds of atomic power plants, but the best known is the Pressurized Water Reactor. This long name is usually abbreviated to PWR.

The PWR isn't really very different from Fermi's pile in Chicago. There is the same big stack of atomic fuel--usually uranium--with control rods sticking out of holes in the fuel bars. Just like Fermi's pile, when the control rods are pushed in they soak up the flying neutrons and there is no reaction. When the control rods are pulled out, the chain reaction takes place.

One of the curious things about a chain reaction is that it won't work if the neutrons are flying too fast. They hit the new atoms at such great speed that they just bounce off and keep going. In order for the neutrons to do their splitting job, they have to be slowed down. Fermi used graphite bricks for this purpose. The PWR uses water, which works very well. And the water also serves another purpose. It absorbs the great heat which is formed in the reactor.

Now, everyone knows that when water is heated to a high temperature, it boils. But _this_ water must not boil. To prevent its boiling, the water is kept under very high pressure, and that is how the Pressurized Water Reactor got its name.

The water is sealed in special tubes and reaches a temperature of about 600° F. The tubes then heat _other_ water which turns into steam.

A simpler kind of atomic power plant is the Boiling Water Reactor, or BWR. The BWR is just a tank which holds a reactor and water. In this case, the water is _not_ under pressure and the heat released by the chain reaction makes it boil. The steam which comes from the boiling water goes directly to the turbine.