Compound
Compound, in chemistry, a substance composed of atoms of two or more elements in chemical combination, occurring in a fixed, definite proportion and arranged in a fixed, definite structure. A compound is often represented by its chemical formula. The formula for water is H 2 O, and for sodium chloride, NaCl. The formula weight of a compound can be determined from its formula. The molecular weight of a molecular compound can be determined from its molecular formula. Two or more distinct compounds that have the same molecular formula but different properties are called isomers. Formation and Decomposition of Compounds Compounds are formed from simpler substances by chemical reaction . Some compounds can be formed directly from their constituent elements, e.g., water from hydrogen and oxygen: 2H 2 + O 2 → 2H 2 O. Other compounds are formed by reaction of an element with another compound; e.g., sodium hydroxide (NaOH) is formed (and hydrogen gas released) by the reaction