Compounds

Compounds are substances composed of two or more elements that are chemically bonded in fixed proportions. In a compound, the elements lose their individual identities in favor of a new substance with distinct properties. Water (H2O) and sodium chloride (NaCl) are common examples; the properties of these compounds, such as boiling point and reactivity, differ markedly from hydrogen, oxygen, sodium, or chlorine alone.

Compounds form through chemical bonds. The main types are ionic, covalent, and metallic. Ionic compounds arise from electrostatic attractions between oppositely charged ions, typically formed when a metal transfers electrons to a nonmetal. Covalent compounds result from shared electron pairs between atoms. Metallic compounds consist of lattices of atoms with delocalized electrons. Many substances may contain both ionic and covalent character.

Compounds have fixed compositions; their formulas indicate the simplest ratio of constituent elements, and molecular formulas show the exact numbers of atoms in a molecule. Some compounds exist as crystalline solids with repeating unit cells; others are liquids or gases. Because of their defined structures, compounds generally have properties that are predictable to an extent from their composition and arrangement, but this can vary with phase, temperature, and pressure.

Compounds form and break through chemical reactions. They can be synthesized by combining elements or simpler compounds, and they can be decomposed into their constituents by chemical means such as electrolysis or thermal decomposition. Separation into pure elements is not achieved by physical methods alone; chemical methods are required.

Representation and naming follow standardized systems. Chemical formulas express composition, while systematic names describe structure and bonding. The study of compounds encompasses inorganic and organic chemistry, materials science, geology, and biochemistry, reflecting the central role compounds play in chemistry and biology.