Jana MacGregor, age 16, of Glendale, Ariz., for her. question:

IN CHEMISTRY, WHAT IS VALENCE?

Valence is a number that indicates the ability of a chemical element to combine with other elements. In the past, valence had several slightly different meanings. The term is now generally being replaced by more precise chemical descriptions.

Valence was first defined as the number of hydrogen atoms that can combine with each atom of an element. For example, each atom of oxygen can be combined with two hydrogen atoms to form water. Therefore, 'oxygen had a valence of two.

A second definition of valence is based on the changes of ionized atoms. Sodium ions have one positive charge, so the valence of sodium is one.

A third definition is based on the number of bonds or chemical links that an atom forms with other atoms. Carbon atoms usually form four bonds, as in the compound methane. As a result, carbon is said to have a valence of four.

Many elements can combine in so many ways that they have several valences. Sulfur has common valences of 2, 4 and 6.