Ions
For ions, the charge on a particular atom may be denoted with a right-hand superscript. For example Na+, or Cu2+. The total charge on a charged molecule or a polyatomic ion may also be shown in this way. For example: H3O+ or SO42−.
For more complex ions, brackets are often used to enclose the ionic formula, as in 2−, which is found in compounds such as Cs2. Parentheses ( ) can be nested inside brackets to indicate a repeating unit, as in 3+. Here (NH3)6 indicates that the ion contains six NH3 groups, and encloses the entire formula of the ion with charge +3.
Read more about this topic: Chemical Formula