What the numbers in a formula mean

The formula of an ion can contain both subscript and superscript numbers. Chemists do not bother with the number 1, it is implied by the absence of other numbers. For example the hydroxide ion is written OH-: it contains 1 oxygen atom, 1 hydrogen atom and has a charge of -1. (It is not written OH1-.) Water is written H2O, not H2O1.

A subscript number gives the number of the atom preceding it. For example H2O contains 2 hydrogen particles and 1 oxygen particle. If the number is preceded by a bracketed group of atoms then the number multiplies all the atoms within the brackets. For example, Mg(OH)2 contains 1 Mg particle, 2 oxygen particles and 2 hydrogen particles.

The superscript gives the charge on an ion. For example O2- has a charge of -2.


The formula of the sulfate ion is SO42-. This tells us that it consists of 1 sulfur particle, 4 oxygen particles and it has a charge of -2.