Terri M. Ratliff, age 10, of Montgomery, Alabama, for her question:

Will you please explain a neutron?

The neutron takes us down to a realm that is even smaller than the minuscule atom. We have to strain our brains to imagine its tiny size. It takes millions of average sized atoms to measure one inch. Naturally the tiny atom is too small for our eyes to see. And inside that atom there may be 10, 50 or even 100 or more neutrons. The tiny neutron is an amazing particle of matter, and our clever scientists found it and figured out what it is like.

We know, of course, that all the materials in our world are made from invisible tiny atoms. The number of atoms in your pencil is in the trillions. And each atom is built from even tinier particles of matter    protons, electrons and neutrons. These atomic particles, or most of them, have minuscule charges of electricity. So you wonder why you don't get an electric shock when you hold a pencil  ¬maybe to write a question to Andy. This does not happen due to the way the atom is built    and one of its building blocks is the neutron.

In the realm of atomic particle s, there are two opposite kinds of electricity  positive and negative. In some ways, they act like the opposite poles of a magnet. All the particles called protons have exactly the same amount of positive electricity, and all the electron particles have an equal and opposite charge of negative electricity. A normal atom is built somewhat like a miniature solar system, with a central sun and orbiting planets. Its central nucleus is packed with a set number of positive protons and orbited by the same number of negative electrons.

Usually, the number of protons and electrons is equal and evenly balanced so that they cancel out their opposite charges. Pencils and other materials do not jolt us with shocks because their atoms have become electrically neutral.

The neutron is a neutral particle, neither positive nor negative, and it is a separate particle of matter from both the proton and the electron in its neutral state. However, the neutron is formed by the combining of one positive proton and one negative electron, thus cancelling out the electric charges and becoming neutral. An atom's neutrons are packed in its tight fisted nucleus with its protons. The nucleus has a set number of protons but its number of neutrons may vary.

Hydrogen and uranium are two of the 100 or so basic atomic elements. Atoms come in different sizes because each element has its own set number of protons. Hydrogen, the smallest atom, has only one proton. Uranium has 92. Every nucleus with one proton is hydrogen. Usually it has no neutron, though rare hydrogen atoms have one or two or even three neutrons. The other elements add protons, one by one, and have more and more neutrons as their atoms grow bigger. All helium atoms have two protons, plus at least one and sometimes two or three neutrons. The huge uranium atom has 92 protons, plus 100 or more neutrons.

Tracking down the atomic particles was a hard job because they are so small. But moving protons and electrons swerve. to opposite poles of a magnetic field. Their tell¬tale trails glow in misty vapors. Neutral neutrons refuse to swerve, but an extra neutron adds extra weight to an atom. It was first discovered in certain overweight hydrogen atoms. They were heavy hydrogen atoms, with the usualy lone hydrogen proton plus an unexpected neutron.