Betty Zinkann, Age 13, of Aurora, Ill., for her question:

What is heavy water?

There has always been heavy water in the world, but its human interest story did not begin until 1932. In that year, the isotope called heavy hydrogen was discovered and heavy water was isolated from ordinary water. The world of science saw new horizons. Heavy water was used in nuclear weapons, and in 1957 it was used in an atomic pile.

Water, of course, is made of molecules, and each mo1ecule has one oxygen and two hydrogen atoms. The hydrogen in heavy water is an isotope called heavy hydrogen or deuterium. Most atoms have a few brother isotopes which are somewhat heavier than themselves. The deuterium isotope is twice as heavy as the ordinary atom of hydrogen.

Hydrogen is the smallest and lightest of all the atoms. Its atomic number is one, which means it has but one positively charged proton in its nucleus. Its nucleus is orbited by only one negatively charged electron. Its weight or mass number is 1.008. The weight of heavy hydrogen is 2.0147. The extra weight comes from a neutron in the nucleus of this isotope atom.

The neutron has no electric charge, and, like hydrogen, the isotope atom has only one proton and one electron. In nature, there is about one atom of the isotope deuterim in every 7000 atoms of ordinary hydrogen. This isotope was isolated in 1932 by Dr. Harold Urey. His work proved the existence of another atomic particle, the neutron. It gave us heavy hydrogen and heavy water, and it opened up a whole new world for chemists and physicists.

Heavy water, in which the hydrogen is the isotope deuterium, may be called deuterium oxide. It freezes at 3.82 degrees Centigrade and boils at 101.42 degrees. Its most important work is done in nuclear physics.

Deuteritml oxide made possible the nuclear reactor in which atoms are split to yield energy by nuclear fission. It acts as a moderator to reduce the speeding neutrons and hence control their energy. It is the coolant that takes heat from the seething core of the atomic pile. This heat can be let off to generate steam and electric power.

In 5000 molecules of ordinary water, there is one molecule of heavy water. This stray particle of deuterium oxide does us no harm. But if all the water in the world became deuterium oxide, life as We know it would come to an end. Seeds cannot sprout in heavy water, and there would be no more plants. Tadpoles cannot live in heavy water, and other animals cannot use it as a substitute for ordinary water.