Formulae of Ions

The charge of an ion is given in superscript in its formula whereas the numbers of each type of atom are subscript. For example SO₄^2- consists of 1 sulphur atom, 4 oxygen atoms and has a 2- charge.

Cations

Simple positive ions are formed by hydrogen and the metals. A positive ion (cation) made of one atom has the same name as its element.

For groups 1-3, the amount of positve charge is the same as the group number. A transition metal often can form several different ions and the amount of positive charge on its ion is given in roman numerals in the compound's name: for example iron (III) chloride contains Fe³⁺ ions whereas iron (II) chloride contains Fe²⁺ ions.

Simple anions

Simple negaive ions (anions) are formed from non-metals. The name of a simple anion starts with the first part of the element's name and ends with -ide. The amount of charge = (group number - 8).

Molecular anions

These are negative ions consisting of several atoms. All the examples below contain oxygen and another element's atoms. The presence of oxygen atoms is indicated by the endings -ite or -ate. There is more oxygen in an '-ate' than in the corresponding '-ite'. When a negative ion is combined with a H+, the compound is an acid. The formulas of these ions are not predictable from the periodic table and they need to be learnt.