Gina Pickens, age 13, of Tucson, Ariz., for her question:

WHAT IS CHEMISTRY?

Chemists use a symbol of one or two letters to stand for each element. For example, the symbol for hydrogen is H and the symbol for helium is He. Some chemical symbols are taken from other languages. The symbol for iron is Fe, taken from ferrum, the Latin word for iron.

Chemistry is the science that deals with or investigates the composition, properties and transformations of substances and various elementary forms of matter.

The fundamental ideas of chemistry deal with the nature of elements and the ways in which elements combine to form compounds.

A knowledge of physical and chemical properties helps scientists analyze substances and classify them into five main groups: elements, compounds, solutions, suspensions and mixtures.

Everything in the world is either an element or a combination of elements. Elements themselves are not combinations of any other substance. Each element is made up of tiny particles of matter called atoms. The atoms of one element differ from those of all other elements. Atoms are made up of even tinier particles of matter.

Chemistry tells us that the atoms of all elements contain the same kinds of particles. Protons and neutrons make up the nucleus or central core of the atom. Protons are tiny positively charged bits of matter. Neutrons are uncharged particles about the same weight as protons. Negatively charged electrons revolve around the nucleus. It takes about 1,840 electrons to equal the weight of one proton.

Atoms of different elements contain different numbers of particles. As an example, you will find one proton and one electron in a hydrogen atom while a uranium atom has 92 protons, 146 neutrons and 92 electrons.

Compounds are formed when different elements combine. Chemistry will show you that water is a type of compound where two atoms of hydrogen are joined with one atom of oxygen to form a molecule of water. A molecule is the smallest part of a substance that has the chemical properties of the substance.

Chemistry is actually the study of substances. The chemist wants to know what substances are made of, how they act and how they change. He wants to find out why chemical changes take place and how they can be controlled.

With the knowledge of chemistry, scientists have developed atomic energy, rocket fuels and chemical devices that produce electricity.