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 SO42- consists of 1 sulphur atom, 4 oxygen atoms and has a 2- charge.


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.

Cation name Periodic table group of element from which cation is made Cation symbol
Hydrogen   H+
Lithium 1 Li+
Sodium 1 Na+
Potassium 1 K+
Magnesium 2 Mg2+
Calcium 2 Ca2+
Aluminium 3 Al3+
Tin(II) and tin(IV) 4 Sn2+ and Sn4+
Lead(II) and lead(IV) 4 Pb2+ and Pb4+
Iron(II) and iron(III) transition Fe2+ and Fe3+
Copper(I) and copper(II)
Copper(II) is more commonly encountered than copper(I).
transition Cu+ and Cu2+
Zinc   Zn2+
Silver transition Ag+
Ammonium   NH4+

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).

Name of element from which anion is formed. Periodic table group of element Anion name Anion symbol
Oxygen 6 Oxide O2-
Sulfur 6 Sulfide S2-
Fluorine 7 Fluoride F-
Chlorine 7 Chloride Cl-
Bromine 7 Bromide Br-
Iodine 7 Iodide I-
Nitrogen 5 Nitride N3-
Phosphorus 5 Phosphide P3-

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.

Molecular anion name Formula Name and formula of the acid containing the anion
Carbonate CO32- Carbonic acid, H2CO3 (unstable, decomposes into H2O and CO2)
Hydrogencarbonate HCO3- Carbonic acid, H2CO3 (unstable, decomposes into H2O and CO2)
Nitrate NO3- Nitric acid, HNO3
Nitrite NO2- Nitrous acid, HNO2
Sulfate SO42- Sulfuric acid, H2SO4
Hydrogensulfate HSO4- Sulfuric acid, H2SO4
Sulfite SO32- Sulfurous acid, H2SO3
Phosphate PO43- Phosphoric acid, H3PO4
Hydroxide OH- (Water, H2O)
Ethanoate CH3COO- Ethanoic acid, CH3COOH
Thiosulfate S2O32- (unstable)