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An ion is an electrically charged atom or molecule (group of atoms). It is "charged" so it will move near electricity. Atoms are made from positively charged protons, negatively charged electrons, and uncharged neutrons. Ions are charged because they have an unequal number of protons and electrons.

Making an ion from an atom or molecule is called ionization. Two or more ions can combine to make a chemical compound. The link between the ions is called an ionic bond.

The charge on a proton is chosen as +1 (positively charged). The charge on an electron is opposite to the charge on the proton. The charge on the electron is -1 (negatively charged). An atom that is ionized makes two parts, one positive, and one negatively charged. For example, a neutral hydrogen atom has one proton and one electron. Ionizing 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.

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 NH+
Hydronium H3O+
Mercury(I) Hg2+
Common anions
Formal name Formula Alt. name
Simple anions
Azide N
Bromide Br
Chloride Cl
Fluoride F
Hydride H
Iodide I
Nitride N3−
Oxide O2−
Sulfide S2−
Carbonate CO2−
Chlorate ClO
Chromate CrO2−
Dichromate Cr2O2−
Dihydrogen phosphate H2PO
Hydrogen carbonate HCO
Hydrogen sulfate HSO
Hydrogen sulfite HSO
Hydroxide OH
Hypochlorite ClO
Monohydrogen phosphate HPO2−
Nitrate NO
Nitrite NO
Perchlorate ClO
Permanganate MnO
Peroxide O2−
Phosphate PO3−
Sulfate SO2−
Sulfite SO2−
Superoxide O
Thiosulfate S2O2−
Silicate SiO4−
Metasilicate SiO2−
Aluminium silicate AlSiO
Anions from organic acids
Acetate CH3COO ethanoate
Formate HCOO methanoate
Oxalate C2O2−
Cyanide CN

Ion Media


  1. 1.0 1.1 Brescia, Frank; Arents, John; Meislich, Herbert; Turk, Amos (1966). Fundamentals of Chemistry: A Modern Introduction (First ed.). Academic Press. p. 5.