Chemical formula: definition in chemistry
Chemical formula: A way of writing a substance with element symbols and subscript numbers that show which elements it contains and in what proportions.
Each subscript gives the number of atoms of the element just before it, and a missing subscript means one, so H₂O has two hydrogen atoms and one oxygen atom. Parentheses group atoms that repeat, as in Ca(OH)₂, which contains two hydroxide groups. A subscript is different from a coefficient: 2H₂O means two water molecules, while changing a subscript would describe a different substance.
Examples
- NaCl — one sodium for every chlorine
- CO₂ — one carbon atom and two oxygen atoms
- Ca(OH)₂ — one calcium, two oxygen and two hydrogen atoms
- C₂H₅OH — ethanol, written to show its OH group
On the periodic table
The symbols used in formulas, such as Na, Cl and Fe, are the one- or two-letter element symbols shown in each box of the periodic table.