compound

A chemical compound is a substance formed when two or more elements are chemically bonded in fixed proportions. Compounds differ from mixtures because their constituent elements are chemically combined and present in defined ratios, resulting in a substance with properties distinct from its elements.

Compounds are produced through chemical reactions that rearrange atoms to create new substances. Once formed, compounds generally maintain their composition and can only be separated into their elements or simpler compounds by chemical changes, such as decomposition or electrolysis.

Chemical formulas express the composition of compounds, for example H2O, NaCl, and CO2. The law of definite proportions states that a given compound contains its elements in constant proportions by mass. Some compounds are molecular, held together by covalent bonds, while others are ionic, composed of a lattice of ions, and still others may be metallic or involve more complex bonding.

Common examples include water (H2O), sodium chloride (NaCl), carbon dioxide (CO2), methane (CH4), and glucose (C6H12O6). Names follow systematic conventions (IUPAC) that reflect composition and structure.

In contrast to elements, which consist of a single type of atom, and mixtures, which are combinations of substances not bonded chemically, compounds are pure substances with specific chemical formulas and properties. The study of compounds encompasses their synthesis, structure, bonding, and reactions, forming a central part of chemistry and its applications in medicine, materials science, and biology.