Compound is a substance that contains more than one kind of atom. Every compound has a definite composition that can be described by a chemical formula. For example, water is a compound that contains two kinds of atoms, hydrogen and oxygen. Other familiar compounds include salt and sugar. These compounds and numerous others occur in nature. Many other compounds are artificially created.
There are more than 100 chemical elements. The atoms in one element are different from those in any other element. The atoms combine in many ways to form millions of compounds. In some cases, atoms of the same elements combine in different proportions to produce a large number of compounds. For example, atoms of carbon and hydrogen can combine to form methane, which is the main component in natural gas. These same elements also form propane, which is used as a fuel for torches and camping stoves. There are thousands of other compounds that contain only carbon and hydrogen.
Each compound has its own distinctive properties. Compounds may be solids, liquids, or gases. They also may have a variety of colors. Some compounds will readily undergo a chemical reaction, but others have little tendency to react.
Compounds can be divided into two groups. Organic compounds contain carbon atoms. Proteins, fats, carbohydrates, nucleic acids, and many other compounds in living things are organic compounds. All other compounds are called inorganic compounds.
Many substances that contain atoms from more than one element are mixtures, not compounds. A compound always has the same composition by weight. However, the composition of a mixture is not fixed and varies from sample to sample. For example, chocolate chip ice cream is a mixture. Its composition varies, and some samples contain more chocolate chips than others.
Chemists prepare compounds in several ways. Some compounds are formed by combining elements. The properties of a compound differ from those of the elements from which it was made. For example, the elements sodium and chlorine combine to form the compound sodium chloride, or table salt. Sodium is a soft metal that reacts violently with water and other substances. Chlorine is a yellowish gas that is poisonous. In contrast, sodium chloride is a hard, unreactive, white, crystalline solid.
Compounds can also be made from other compounds. Living things have the ability to add compounds together to form more complex compounds, and to break down compounds into simpler substances. In addition, many compounds break up into simpler compounds or elements when they are heated to high temperatures or exposed to electric current.
