Janice Fletcher, age 13, of Kershaw, S.C., for her question:

How big is an atom?

The earth, oceans and the air, the sun, moon and all the stars are made of tiny atoms ‑ too small for our eyes to see. Most of the everyday things around us are compounds of assorted atoms. Common salt is a compound of sodium atoms and chlorine atoms. It is made of molecules, each of which is a tightly knit bundle of one sodium atom and one chlorine atom. Is each grain of salt a molecule? Not at all. Salt contains more salt molecules than there are stars in the Milky Way ‑ and each molecule is made from two atoms.

Magnified under a strong microscope, a grain of salt is a pretty sight indeed. It looks like a lump of ice cubes of various sizes, all frozen together in a glassy mass. Experts estimate that this salt crystal is mode from tin million, million, million, million molecules. You could write the number of atoms in a single grain of salt with the number 20 followed by 24 zeros. No wonder an atom is too small for our eyes to see.

Until lately, we had no instrument powerful enough to take a picture of an atom. In fret, the experts had just got around to saying that this could never be done. An atom, they said, is smaller than light waves, and therefore too small to be seen or photographed by any microscope. But, in the Age of Science, it seems nothing is impossible. In 1957, a new kind of microscope was used to photograph individual atoms for the first time.

The impossible job was done by Dr. Erwin Mueller at the University of Pennsylvania. The machine used was a field ion microscope which magnifies two million times ‑ and no, Andy does not know how it works. The pictures are fascinating and some of them may be seen in a wonderful book called Elements of the Universe, by Dr. Glenn Seaborg.

This is a book which every young person interested in physics will want to read ‑ it is tops and written so that you can understand it.

A picture of tungsten, magnified two million times, shows clusters of atoms and individual atoms as bright dots arranged in a lacy doily design. Since the atoms are merely shown as bright dots, we do not yet have a clear portrait of an atom but we do have pictures to prove that individual atoms do exist. For more definite pictures, we must perhaps wait for more powerful instruments to be perfected.

There are, of course, about 90 different atoms in the world around us. The scientists have found a few more varieties in the laboratory. Each king of atom has its own size,, weight and number of particles„ Hydrogen is the lightest, uranium one of the heaviest. Let's try to grasp the size of an average atom, the iron atom, for after all we live in the Iron Age. Suppose that everyone in North America were reduced to the size of an iron atom. What midgets we would be ‑ if we all stood in a row we would measure about two inches. For it takes about 100 million atoms of iron to measure one inch.