A chemical compound is a foundational concept in the study of matter. Compounds are the organized structures that make up everything from the air we breathe to the cells in our bodies. They are formed when two or more separate substances combine to create something entirely new. The predictable nature of how these substances combine allows scientists to analyze, synthesize, and manipulate matter with precision.
Defining the Chemical Compound
A chemical compound is a substance composed of two or more different elements chemically bonded together in a fixed ratio. This adherence to a specific, unchangeable proportion is known as the Law of Definite Proportions. This fixed ratio grants a compound its unique identity, regardless of how it was created. For example, water is always H2O, meaning every molecule contains exactly two hydrogen atoms and one oxygen atom. If the ratio changes, such as in H2O2 (hydrogen peroxide), the result is a completely different compound with distinct properties.
How Compounds Are Formed
The formation of a compound involves a chemical reaction where atoms are held together by chemical bonds, created by the interaction of valence electrons. The two primary types of bonds are ionic and covalent.
Ionic bonds typically form between a metal and a non-metal, involving the complete transfer of electrons to create oppositely charged ions. For instance, in table salt (NaCl), the strong electrostatic force between the positive sodium ion and the negative chloride ion holds the compound together.
Covalent bonds usually occur between two non-metals and involve the sharing of electron pairs between atoms. This sharing allows each atom to achieve a stable, filled outer electron shell. Water (H2O) is a common example, where the oxygen atom shares electrons with two hydrogen atoms.
Distinguishing Compounds from Elements and Mixtures
Compounds are distinct from elements and mixtures, the other major classifications of matter. An element is the simplest form of matter, consisting of only one type of atom, such as pure gold (Au). When elements form a compound, they lose their individual properties to create a substance with entirely new characteristics. For example, highly reactive sodium and poisonous chlorine gas bond chemically to form harmless, stable sodium chloride (table salt).
A compound is fundamentally different from a mixture, which is a physical combination of substances that are not chemically bonded. In a mixture, the components retain their original properties and can be separated by physical means, such as filtration or evaporation. Mixtures also have variable compositions. By contrast, a compound requires a chemical reaction to form, has a fixed ratio of elements, and can only be separated back into its elements through another chemical reaction.
Major Categories of Compounds
Compounds are broadly classified into two major categories: inorganic and organic, based primarily on their elemental composition. Inorganic compounds generally lack carbon-hydrogen bonds and include substances derived from non-living sources, such as minerals and salts. Water (H2O), common table salt (NaCl), and most metals and their oxides fall into this category.
Organic compounds are defined by the presence of carbon atoms, typically bonded to hydrogen atoms, forming the basis of the chemistry of life. These compounds can range from simple molecules like methane (CH4) to the complex structures of sugars, proteins, and DNA. The ability of carbon to form four stable bonds and create long chains and rings allows for the immense diversity seen in organic matter.