It was not until knowledge about atoms and elements was gained that the make-up of the millions of different substances around us was understood. All the scientific knowledge we have today indicates that these different substances are made from only 92 different kinds of atoms that make up the naturally occurring elements.

These atoms are able to join together in millions of different combinations and arrangements to form all the substances in the universe. A molecule is formed when two or more of the same or different kinds of atoms join together. The newly formed substance is called a compound. Although the identity of the elements stays the same because the number of protons is the same, the physical and chemical properties of the compound are different from the properties of the elements from which they formed because the arrangement of the outermost electrons is different.

For centuries chemists...