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 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
Other Languages
Afrikaans: Ioon
العربية: أيون
asturianu: Ion
azərbaycanca: İon
Bân-lâm-gú: Lī-chú
беларуская: Іон
беларуская (тарашкевіца)‎: Іён
български: Йон
bosanski: Ion
brezhoneg: Ion
català: Ió (àtom)
čeština: Ion
Cymraeg: Ïon
dansk: Ion
Deutsch: Ion
eesti: Ioon
Ελληνικά: Ιόν
English: Ion
español: Ion
Esperanto: Jono
euskara: Ioi
فارسی: یون
français: Ion
Gaeilge: Ian
Gaelg: Eeane
galego: Ión
한국어: 이온
հայերեն: Իոն
हिन्दी: आयन
hrvatski: Ion
Ido: Iono
Bahasa Indonesia: Ion
interlingua: Ion
Ирон: Ион
italiano: Ione
עברית: יון
Basa Jawa: Ion
ಕನ್ನಡ: ಅಯಾನು
ქართული: იონი
Kreyòl ayisyen: Anyon
kurdî: Îyon
Кыргызча: Иондор
Latina: Ion
latviešu: Jons
lietuvių: Jonas (dalelė)
Limburgs: Ion
lumbaart: Jun
magyar: Ion
македонски: Јон
Malagasy: Rio
മലയാളം: അയോൺ
मराठी: आयन
Bahasa Melayu: Ion
монгол: Ион
မြန်မာဘာသာ: အိုင်ယွန်
Nederlands: Ion (deeltje)
日本語: イオン
Nordfriisk: Ion
norsk: Ion
norsk nynorsk: Ion
Novial: Ione
occitan: Ion
oʻzbekcha/ўзбекча: Ion
ਪੰਜਾਬੀ: ਆਇਨ
پنجابی: آئن
Plattdüütsch: Ion
polski: Jon
português: Íon
română: Ion
Runa Simi: Iyun
русиньскый: Йон
русский: Ион
Scots: Ion
shqip: Joni
sicilianu: Ioni
සිංහල: අයනය
slovenčina: Ión (častica)
slovenščina: Ion
српски / srpski: Јон
srpskohrvatski / српскохрватски: Jon
Basa Sunda: Ion
suomi: Ioni
svenska: Jon
Tagalog: Iono
தமிழ்: அயனி
తెలుగు: అయాన్
ไทย: ไอออน
тоҷикӣ: Ион
Türkçe: İyon
українська: Іон
اردو: آئون
vepsän kel’: Ion
Tiếng Việt: Ion
文言: 離子
Winaray: Iono
吴语: 离子
Yorùbá: Íónì
粵語: 離子
中文: 离子