Roberta Johnson, Age 7, Of New York, N. Y., for her question:

What is the heaviest element?

Roberta already knows that hydrogen is the lightest of the elements. One atom of oxygen is about 16 times heavier than an atom of hydrogen, and oxygen also is one of the lightweight elements. An atom of gold is 197 times heavier than an atom of hydrogen. Certain atoms of uranium are 238 times heavier than atoms of hydrogen.

Uranium is one of the heaviest elements found in the earth's crust. But scientists have found traces of still heavier elements. But all of these big, heavy atoms are radioactive. They are constantly breaking apart into smaller, lighter atoms. Some of them exist only for the smallest part of a second.