John Hanson, age 12, of Sioux Falls, S.D., for his question:

WHAT IS OZONE?

Ozone is a form of oxygen. Its sharp odor is often noticed near electric switches and machinery, and in the air after a thunderstorm. It was discovered by a German chemist named Christian Friedrich Schonbein in 1840.

Molecules of oxygen are made of two oxygen atoms joined tightly together. But ozone molecules have a third oxygen atom loosely attached to these two. The third atom can easily separate from the molecule and combine with other substances. As a result, ozone is a chemically active gas.

On the average, air at ground level contains less than one part ;of ozone per million parts of air. But it still shields the earth from much of the sun's ultraviolet light.