Anson Barnett, age 16, of Harrisburg, Penn., for his question:

WHAT IS THE LIGHTEST KNOWN METAL?

Lithium is a soft, silvery white metallic element that is the lightest known metal. It is only half as heavy as an equal volume of water.

Lithium reacts with water, as does sodium, to release hydrogen gas. But unlike sodium, the reaction does not ignite the hydrogen.

Lithium is used to make certain types of batteries. Lithium compounds are used in the manufacture of various materials, including ceramic products, enamels, glass and lubricants, for use at high temperatures. They also are used in rubber products and in dyes for textiles.

Lithium metal does not occur in nature, but combines easily with other elements. Chemists obtain the metal by passing electricity through fused or melted lithium chloride.

Lithium melts at 354 degrees Fahrenheit and boils at 2,403 degrees.

A Swedish chemist named Johann Arfvedson discovered lithium in 1817.