Allen Lyturyn, age 13, of Dauphin, Manitoba, Canada, for his question:

What elements are present in the human body?

Chemical elements, of course, are composed of infinitesimal units of matter. The physical human body is built entirely of these assorted atoms and molecule packages of atoms. All atoms of an element are alike and all molecules of a chemical compound are alike. The number of elements in the human body is limited, the assortment of compounds and chemical mixtures is almost uncountable.

Nature uses only 92 chemical elements to make the uncountable number of solids, liquids and gases in the entire world. Oxygen is so plentiful that it makes up almost half the total bulk of earthly substances. Precious gold makes up less than a millionth part of the earth's crust, and rare argon makes up less than a hundred thousandth part of the gaseous atmosphere. A few atoms of francium exist briefly during the decay of certain radioactive substances. You might expect the miraculous human body to be made entirely of the rarest of elements, or at least the elements we rate as precious or valuable. Not at all.

The cash value of the elements in the human body amounts to only a few dollars. The main ingredient is oxygen, the most plentiful of all elements. The red blood cells contain a shifting quota of oxygen gas. As it performs its duties, its atoms combine with carbon to form molecules of carbon dioxide. We count the oxygen ingredient in this compound in our list of body elements. We also count the oxygen combined with hydrogen in water molecules    and the body contains a lot of liquid solutions. Water makes up almost 70 per cent of its total weight. When we tot up all the free and combined oxygen we find that this common element makes up about 65 per cent of the entire body

The next most plentiful element ingredient is carbon. It makes up 18 per cent  of the body. Hydrogen makes up ten per cent and nitrogen three per cent. Calcium  makes up two per cent and phosphorus one per cent. The percentages of these six  elements add up to 99 per cent. The remaining one per cent is shared by more than  20 other elements. Seven of these are present in amounts large enough to be identi¬  fied easily. They are potassium and sulphur, sodium and chlorine, magnesium, iron   and iodine. The first two account for most of the last one per cent. A list of 15  or so elements are present in mere traces. It includes smidgens of silicon and lithium, fluorine and rubidium and traces of well known metals such as aluminum and copper, nickel and zinc, chromium and silver and even gold.

Some of the trace elements are present in salts and other compounds, just as most of the body's oxygen is present as an ingredient of water. Biologists know the vital functions of more than 30 elements in the human body. But stray atoms of other elements also are present. Traces of radioactive atoms exist everywhere, even in the human body. As they decay, they break into smaller atoms. At this moment, your body may or may not contain a stray atom of the rare element radon or francium.

You most likely know something about the roles of oxygen and the body's other most abundant elements. But those present in traces also perform vital functions in the life processes. The thyroid gland needs an amount of iodine equal to 0.00004 of one per cent of the total body to perform the vital balancing processes of chemical metabolism. The muscular heart needs smidgens of magnesium and potassium. The digestive system cannot perform its chemical miracles without the help of small amounts of zinc and chlorine.