Chris Miller, age 11, of Edison, New Jersey, for his question:

What is the lightest substance in the world?

There is plenty of the lightest substance around, but most of it is tied up behind the scenes. This is fine because when alone and free to roam it is hot tempered stuff that tends to explode in flames. When combined with other substances, it usually becomes safe, everyday molecules. There are countless tons of this substance tied up in the waters of the oceans and there's a small helping in every raindrop. Nore is tied up in other molecules needed by living cells. x

The world's lightest substance is hydrogen, chemical element number one. On earth, eight other elements are more plentiful    but in the rest of the universe, these eight are rather scarce. Out in space, hydrogen is king. It is the main ingredient in the sun, the stars and the great clouds of cosmic dust strewn through the Galaxy. Astronomers suspect that everything in our Solar System may have been formed from an original cosmic cloud made mostly of hydrogen gas.

When our world was born, there was a lot of hydrogen in its enormous atmosphere. But because atoms of hydrogen are so small and light, most of them managed to escape the pull of the earth's atmosphere and lost themselves in space. Nowadays hydrogen mattes up only one per cent of our atmosphere  ¬and most of these atoms are locked safely fn molecules of water vapor.

In a few places, pockets of hydrogen gas are trapped underground, with deposits of petroleum and natural gas. At one time, it was used in balloons because hydrogen gas is 14 times lighter than air. However, it is highly explosive. tfany hydrogen filled balloons and dirigibles were destroyed in disastrous fires. Nowadays, these lighter than air craft are lifted by helium gas. Helium is twice as heavy as hydrogen but 100 per cent safer.

Hydrogen has atomic number one because there is only one proton in the nucleus of the hydrogen atom. No other chemical element can make this claim. One of every 6,000 hydrogen atoms has a nucleus of one proton and one neutron. This is an isotope called heavy hydrogen, or deuterium rarer hydrogen isotope called tritium has a proton and two neutrons. Tritium is slightly radioactive.

All these atoms are hydrogen isotopes because they have but one proton and one orbiting electron. The weight of a proton is one unit    and the atomic weight of hydrogen is 1.00797. This shows that the weight of one little electron is hardly worth mentioning. Since every atom must have at least one proton, the hydrogen atom must be the smallest and the lightest. Hence, no substance in the world can be lighter than hydrogen.

In hydrogen gas, pairs of atoms tend to unite and form hydrogen mole¬cules. These and single hydrogen atoms are eager to combine with all sorts of other elements. For example, two hydrogens and one oxygen unite to form particles of water. Hydrogen combines with carbon and several other common substances to form thousands of different substances. These chemicals in¬clude sugars, starches and numerous other substances in the cells of plants and animals. Hydrocarbons are combinations of hydrogen and carbon alone found in coal and petroleum, in fossils and fossil fuels. In all these chemical compounds, the world's lightest substance is tied up safely behind the scenes.