Janise Meyertons, age 13, of Sayreville, New Jersey, for her question:

Is it true that salt is made of poisons?

Yes, indeed, it is true that ordinary table salt is made from two separate ingredients that are hazardous to human health. But salt is not poisonous and our bodies need a daily quota of it. The safety secret is in the key words "separate ingredients." The wizardry behind the scenes is one example of how two chemicals can combine to create a sub¬stance that is entirely different from either of them.

Let's not give up salt because it is made from a couple of poisonous substances. The finished recipe is perfectly safe and absolutely neces¬sary to human health. The two hazardous ingredients are the chemicals sodium and chlorine. The recipe that changes them into ordinary salt is called a chemical reaction.

Chlorine is a greenish yellow, suffocating gas that irritates eyes, throat and lungs. Sodium is a soft silvery white, hot tempered metal which can burn the skin or even explode. Both sodium and chlorine are chemical elements, each made from its own atoms. The structure of these atoms makes it possible for the two hazardous ingredients to combine and form molecules of safe salt. When this happens, the basic chemical ele¬ments are transformed into a chemical compound.

To understand the operation, we should picture the two basic atoms involved. The atomic number of sodium is 11, which means that its tight¬fisted nucleus contains 11 positively charged protons. Normally this positive charge is neutralized by 11 orbiting electrons, which are nega¬tively charged. Two electrons occupy the first complete shell. Eight electrons complete a second shell, leaving a lone ranger to start a third. Sodium is chemically active because all atoms strive to complete their outside electron shells.

The atomic number of the element chlorine is 17, which means its nucleus has 17 protons, surrounded by 17 swarming electrons. The first and second electron shells are completed with two and eight, leaving seven in the third shell    which needs one more to be complete. Hence, in order to complete their third shells, sodium needs seven more electrons and chlorine needs one.

The problem is solved when one atom of sodium and one of chlorine combine to share their one and seven electrons to form a complete outer shell of eight. To do this, the pair of atoms must lock together and share the same outer shell. No longer are they separate atoms that behave like hazardous sodium and chlorine. The combined pairs are molecules of ordinary salt, with a very different chemical life of its own.

When a sliver of hot tempered sodium is s dropped into a flask of choking sodium gas, pairs of peppy atoms combine by themselves. Soon they form a pinch of ordinary salt. However, the whole thing happens on a sub microscopic scale. The number of tiny molecules needed to build a grain of salt is 10, plus 24 zeros    more or less. The same number of sodium atoms and the same number of chlorine atoms were needed to build it    10 plus 24 zeros of each basic ingredient.