Part 3

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.

WHAT'S IN AN ATOM?

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.