An ion is an electrically charged atom or group of atoms.[1] It is a part of an atom, or part of a group of atoms (molecule). It is "charged" so it will move near electricity. This is because atoms are made of three smaller parts:

  1. neutrons (with no charge)
  2. equal numbers of charged protons and
  3. oppositely-charged electrons.

An ion has unequal numbers of protons and electrons. Making an ion from an atom or molecule is called ionization.

The charge on a proton is measured as +1 (positively charged). The charge on an electron is measured as -1 (negatively charged). An atom that is ionized makes two ions, one positive, and one negatively charged. For example, a neutral hydrogen atom has one proton and one electron. Heating the atom breaks it into two parts: (1) a positively charged hydrogen ion, H+ (2) a negatively charged electron.

A liquid with ions is called an electrolyte. A gas with lots of ions is called a plasma. When ions move, it is called electricity. For example, in a wire, the metal ions do not move, but the electrons move as electricity. A positive ion and a negative ion will move together. Two ions of the same charge will move apart. When ions move they also make magnetic fields.

Many ions are colourless. Elements in the main groups in the Periodic Table form colourless ions. Some ions are coloured. The transition metals usually form coloured ions.


In physics, atomic nuclei that have been completely ionized are called charged particles. These are ones in alpha radiation.

Ionization happens by giving atoms high energy. This is done using electrical voltage or by high-energy ionizing radiation or high temperature.

A simple ion is formed from a single atom.

Polyatomic ions are formed from a number of atoms. Polyatomic ions usually consist of all non-metal atoms. But sometimes the polyatomic ion can have a metallic atom too.

Positive ions are called cations.[1] They are attracted to cathodes (negatively charged electrodes). (Cation is pronounced "cat eye on", not "kay shun".) All simple metal ions are cations.

Negative ions are called anions.[1] They are attracted to anodes (positively charged electrodes). All simple non-metal ions (except H+, which is a proton) are anions (except NH4+).

Transition metals can form more than one simple cation with different charges.

Most ions have a charge of less than 4, but some can have higher charges.

Michael Faraday was the first person to write a theory about ions, in 1830. In his theory, he said what the portions of molecules were like that moved to anions or cations. Svante August Arrhenius showed how this happened. He wrote this in his doctoral dissertation in 1884 (University of Uppsala). The university did not accept his theory at first (he only just passed his degree). But in 1903, he won the Nobel Prize in Chemistry for the same idea.

In Greek ion is like the word "go". "Anion" and "cation" mean "up-goer" and "down-goer". "Anode" and "cathode" are "way up" and "way down".

Common ions

Common cations
Common name Formula Historic name
Simple cations
Aluminium Al3+
Barium Ba2+
Beryllium Be2+
Calcium Ca2+
Chromium(III) Cr3+
Copper(I) Cu+ cuprous
Copper(II) Cu2+ cupric
Hydrogen H+
Iron(II) Fe2+ ferrous
Iron(III) Fe3+ ferric
Lead(II) Pb2+ plumbous
Lead(IV) Pb4+ plumbic
Lithium Li+
Magnesium Mg2+
Manganese(II) Mn2+
Mercury(II) Hg2+ mercuric
Potassium K+ kalic
Silver Ag+ argentous
Sodium Na+ natric
Strontium Sr2+
Tin(II) Sn2+ stannous
Tin(IV) Sn4+ stannic
Zinc Zn2+
Polyatomic cations
Ammonium NHScript error: No such module "Su".
Hydronium H3O+
Mercury(I) HgScript error: No such module "Su". mercurous
Common anions
Formal name Formula Alt. name
Simple anions
Azide NScript error: No such module "Su".
Bromide Br
Chloride Cl
Fluoride F
Hydride H
Iodide I
Nitride N3−
Oxide O2−
Sulfide S2−
Carbonate COScript error: No such module "Su".
Chlorate ClOScript error: No such module "Su".
Chromate CrOScript error: No such module "Su".
Dichromate CrScript error: No such module "Su".OScript error: No such module "Su".
Dihydrogen phosphate HScript error: No such module "Su".POScript error: No such module "Su".
Hydrogen carbonate HCOScript error: No such module "Su". bicarbonate
Hydrogen sulfate HSOScript error: No such module "Su". bisulfate
Hydrogen sulfite HSOScript error: No such module "Su". bisulfite
Hydroxide OH
Hypochlorite ClO
Monohydrogen phosphate HPOScript error: No such module "Su".
Nitrate NOScript error: No such module "Su".
Nitrite NOScript error: No such module "Su".
Perchlorate ClOScript error: No such module "Su".
Permanganate MnOScript error: No such module "Su".
Peroxide OScript error: No such module "Su".
Phosphate POScript error: No such module "Su".
Sulfate SOScript error: No such module "Su".
Sulfite SOScript error: No such module "Su".
Superoxide OScript error: No such module "Su".
Thiosulfate SScript error: No such module "Su".OScript error: No such module "Su".
Silicate SiOScript error: No such module "Su".
Metasilicate SiOScript error: No such module "Su".
Aluminium silicate AlSiOScript error: No such module "Su".
Anions from organic acids
Acetate CHScript error: No such module "Su".COOScript error: No such module "Su". ethanoate
Formate HCOOScript error: No such module "Su". methanoate
Oxalate CScript error: No such module "Su".OScript error: No such module "Su". ethanedioate
Cyanide CN

Related pages

List of ions