Chemistry

WRITTEN BYAlan J. RockeSee All ContributorsProfessor of History, Case Western Reserve University, Cleveland, Ohio, U.S. Author of The Quiet Revolution: Hermann Kolbe and the Science of Organic Chemistry and other books.See Article History

Chemistry, the science that deals with the properties, composition, and structure of substances (defined as elements and compounds), the transformations they undergo, and the energy that is released or absorbed during these processes. Every substance, whether naturally occurring or artificially produced, consists of one or more of the hundred-odd species of atoms that have been identified as elements. Although these atoms, in turn, are composed of more elementary particles, they are the basic building blocks of chemical substances; there is no quantity of oxygen, mercury, or gold, for example, smaller than an atom of that substance. Chemistry, therefore, is concerned not with the subatomic domain but with the properties of atoms and the laws governing their combinations and how the knowledge of these properties can be used to achieve specific purposes.

Development Of Atomic Theory

The concept of the atom that Western scientists accepted in broad outline from the 1600s until about 1900 originated with Greek philosophers in the 5th century BCE. Their speculation about a hard, indivisible fundamental particle of nature was replaced slowly by a scientific theory supported by experiment and mathematical deduction. It was more than 2,000 years before modern physicists realized that the atom is indeed divisible and that it is not hard, solid, or immutable.