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Expression of the composition or structure of a chemical compound. Formulas for molecules use chemical symbols with subscript numbers to show the number of atoms of each element: O for molecular oxygen, O for ozone, CH for methane, CH for benzene. Parentheses may enclose atoms that act as a group. General formulas show the proportions of atoms in members of a class (e.g., CH for alkanes). If the substance does not exist as molecules (seeionic bond), empirical formulas show the relative proportions of the constituents (e.g., NaCl for sodium chloride). Structural formulas show bonds (seebonding) between atoms in a molecule as short lines between symbols; they are particularly useful for showing how isomers differ. A projection formula also indicates the three-dimensional arrangement of the atoms (seeFischer projection; stereochemistry).
This entry comes from Encyclopædia Britannica Concise. For the full entry on chemical formula, visit Britannica.com.
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