Pam Starr Gotcher, age 11, of Bossier, City, La., for her question:

What are molecules?

A molecule is a well tied package of atoms. The atoms in the bundle may be all of one kind or they may be an assortment of different atoms. A molecule of oxygen is composed of two atoms of oxygen, a molecule of ozone is composed of three atoms of oxygen. A molecule of water is one oxygen atom and two hydrogen atoms. A molecule of ammonia is one nitrogen atom and three hydrogen atoms.

A molecule is vastly different from its individual atoms. Water, a liquid, is composed of two gases, hydrogen and oxygen. Crisp white salt crystals are composed of the metal sodium and the gas chlorine. Ammonia is composed of atoms of nitrogen and hydrogen, both gases. The molecule is always different in nature from its ingredients.

In bulk, molecules form compounds. A box of pure salt is a compound. Each small particle is a molecule just like every other salt molecule. There are fewer than 100 different atoms in everyday use. They tie up in molecules, break apart and form others. They form the countless different compounds of our everyday lives.

Molecules are formed by busy electrons. The electrons are atomic particles of negative electricity. They circle the nucleus as the planets circle the sun. Each type of atom has its own quota of electrons. Hydrogen has one, oxygen has eight, nitrogen has seven, sodium has 11, chlorine has 17. The electrons equal the number of positive protons in the nucleus of the atom. Hence, a normal atom is electrically balanced.

The electron shells are neat and orderly. The inner shell is complete with two, the next with eight. These whirling shells are called energy levels because the outer shells have more energy than the inner shells. A jolt of energy may send the outer electrons whirling in wider orbits. Before they subside back to normal they may tangle with electrons from nearby atoms.

Each whirling electron acts like a magnet. And a magnet attracts the opposite pole of another magnet. Electrons then, 1'`,) to work in pairs. What's more, every shell of electrons likes to be complete. The lone electron of the hydrogen atom would like another electron to complete its shell. The six electrons in the second shell of oxygen would like two electrons to feel complete.

So, two hydrogen atoms team up with one oxygen atom. Each hydrogen atom now has two electrons shared with the oxygen. The oxygen now has a complete second shell, two pairs of its own and two pairs shared with the hydrogen atoms. We got a water molecule.

Nitrogen has a complete inner shell of two and five electrons in a second shell. It forms three pairs with three hydrogen atoms and one pair of its own. The four atoms form a molecule of ammonia. Sodium has two complete shells and one electron is a third. Chloride has two complete shells and seven in a third. Sodium lends its lone electron to complete the outer shell of chlorine. The two atoms together form a molecule of salt. Countless molecules join to form a crystal of salt.