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 H2O, 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.
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