In chemistry, an onium ion is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen (group 15 of the periodic table), chalcogen (group 16), or halogen (group 17). The oldest-known onium ion, and the namesake for the class, is ammonium, NH+4, the protonated derivative of ammonia, NH₃.^[1][2]

The name onium is also used for cations that would result from the substitution of hydrogen atoms in those ions by other groups, such as organic groups, or halogens; such as tetraphenylphosphonium, (C₆H₅)₄P⁺. The substituent groups may be divalent or trivalent, yielding ions such as iminium and nitrilium.^[1][2]

A simple onium ion has a charge of +1. A larger ion that has two onium ion subgroups is called a double onium ion, and has a charge of +2. A triple onium ion has a charge of +3, and so on.

Compounds of an onium cation and some other anion are known as onium compounds or onium salts.

Onium ions and onium compounds are inversely analogous to -ate ions and ate complexes:

• Lewis bases form onium ions when the central atom gains one more bond and becomes a positive cation.
• Lewis acids form -ate ions when the central atom gains one more bond and becomes a negative anion.^[3]

• boronium cation, BH+4 (protonated borane)
  • further boronium cations, B
    _xH⁺
    _y (protonated boranes)

• carbonium ions (protonated hydrocarbons) have a pentacoordinated carbon atom with a +1 charge.
  • alkanium cations, C
    _nH⁺
    _2n+3 (protonated alkanes)
    • methanium, CH+5 (protonated methane) (Sometimes called carbonium, because it is the simplest member of that class, but that use is deprecated because of multiple definitions.^[4] Sometimes called methonium, but methonium also has multiple definitions. Abundant in outer space.)
    • ethanium, C₂H+7 (protonated ethane)
    • propanium, C₃H+9 (propane protonated on an unspecified carbon)
      • propylium, or propan-1-ylium (propane protonated on an end carbon)
      • propan-2-ylium (propane protonated on the middle carbon)
    • butanium, C₄H+11 (butane protonated on an unspecified carbon)
      • n-butanium (n-butane protonated on an unspecified carbon)
        • n-butylium, or n-butan-1-ylium (n-butane protonated on an end carbon)
        • n-butan-2-ylium (n-butane protonated on a middle carbon)
      • isobutanium (isobutane protonated on an unspecified carbon)
        • isobutylium, or isobutan-1-ylium (isobutane protonated on an end carbon)
        • isobutan-2-ylium (isobutane protonated on the middle carbon)
    • octonium or octanium, C₈H+19 (protonated octane)
• silanium (sometimes silonium), SiH+5 (protonated silane) (should not be called siliconium^[5]
  • disilanium, Si₂H+7 (protonated disilane)
  • further silanium cations, Si
    _nH⁺
    _2n+3 (protonated silanes)
• germonium, GeH+5 (protonated germane)
• stannonium, SnH+3 (protonated SnH₂) (not protonated stannane SnH₄)
• plumbonium, PbH+3 (protonated PbH₂) (not protonated plumbane PbH₄)
• flerovonium, FlH+3 (protonated FlH₂) (not protonated flerovane FlH₄)

• ammonium (IUPAC name azanium), NH+4 (protonated ammonia (IUPAC name azane))
• phosphonium, PH+4 (protonated phosphine)
• arsonium, AsH+4 (protonated arsine)
• stibonium, SbH+4 (protonated stibine)
• bismuthonium, BiH+4 (protonated bismuthine)
• moscovonium, McH+4 (protonated moscovine)

• oxonium, H₃O⁺ (protonated water (IUPAC name oxidane). Oxonium is better known as hydronium, though hydronium implies a solvated or hydrated proton. It may also be called hydroxonium.)
• sulfonium, H₃S⁺ (protonated hydrogen sulfide)
• selenonium, H₃Se⁺ (protonated hydrogen selenide)
• telluronium, H₃Te⁺ (protonated hydrogen telluride)
• polononium, H₃Po⁺ (protonated hydrogen polonide)
• livermoronium, H₃Lv⁺ (protonated hydrogen livermoride)

• hydrogenonium, better known as trihydrogen cation, H+3 (protonated molecular or diatomic hydrogen), found in ionized hydrogen and interstellar space

• fluoronium, H₂F⁺ (protonated hydrogen fluoride)
• chloronium, H₂Cl⁺ (protonated hydrogen chloride)
• bromonium, H₂Br⁺ (protonated hydrogen bromide)
• iodonium, H₂I⁺ (protonated hydrogen iodide)
• astatonium, H₂At⁺ (protonated hydrogen astatide)
• tennessonium, H₂Ts⁺ (protonated hydrogen tennesside)

• aminodiazonium, [H₂N=N=N]⁺ ⇌ [H₂N−N≡N]⁺ (protonated hydrogen azide)
• methylidyneammonium and hydrocyanonium, H₂CN⁺, isomers HC≡NH⁺ ⇌ N≡CH+2 (protonated hydrogen cyanide)

• hydrohelium or helonium, better known as helium hydride ion, HeH⁺ (protonated helium)
• neonium, NeH⁺ (protonated neon)
• argonium, ArH⁺ (protonated argon)
• kryptonium, KrH⁺ (protonated krypton)
• xenonium, XeH⁺ (protonated xenon)
• radonium, RnH⁺ (protonated radon)
• oganessonium OgH⁺ (protonated oganesson)

• primary ammonium cations, RH₃N⁺ or R−NH+3 (protonated primary amines)
  • hydroxylammonium, H₃N⁺−OH (protonated hydroxylamine)
  • methylammonium, CH₃NH+3 (protonated methylamine)
  • ethylammonium, CH₃CH₂NH+3 (protonated ethylamine)
  • hydrazinium, or diazanium, H₂N−NH+3 (protonated hydrazine, a.k.a. diazane)
  • anilinium (a.k.a. phenylammonium), C₆H₅−NH+3 (protonated aniline, a.k.a. phenylamine, aminobenzene)
• secondary ammonium cations, R₂NH+2 (protonated secondary amines)
  • dimethylammonium (sometimes dimethylaminium), (CH₃)₂NH+2 (protonated dimethylamine)
  • diethylammonium (sometimes diethylaminium), (CH₃CH₂)₂NH+2 (protonated diethylamine)
  • ethylmethylammonium, (CH₃CH₂)(CH₃)NH+2 (protonated ethylmethylamine)
  • diethanolammonium (sometimes diethanolaminium), (OHCH₂CH₂)₂NH+2 (protonated diethanolamine)
• tertiary ammonium cations, R₃NH⁺ (protonated tertiary amines)
  • trimethylammonium (CH₃)₃NH⁺ (protonated trimethylamine)
  • triethylammonium (CH₃CH₂)₃NH⁺ (protonated triethylamine)
• quaternary ammonium cations, R₄N⁺ or NR+4
  • tetrafluoroammonium, NF+4
  • tetramethylammonium, (CH₃)₄N⁺
  • tetraethylammonium, (CH₃CH₂)₄N⁺
  • tetrapropylammonium, (CH₃(CH₂)₂)₄N⁺
  • tetrabutylammonium, (CH₃(CH₂)₃)₄N⁺ or abbreviated Bu₄N⁺
  • trimethyl ammonium compounds, (CH₃)₃RN⁺
  • didecyldimethylammonium, (CH₃(CH₂)₉)₂(CH₃)₂N⁺
  • pentamethylhydrazinium, (CH₃)₂N−N(CH₃)+3
• quaternary phosphonium cations, R₄P⁺ or PR+4
  • tetraphenylphosphonium, (C₆H₅)₄P⁺
• quaternary arsonium cations, R₄As⁺ or AsR+4
  • tetraphenylarsonium, (C₆H₅)₄As⁺
• quaternary stibonium cations, R₄Sb⁺ or SbR+4
  • tetraphenylstibonium, (C₆H₅)₄Sb⁺
• primary oxonium cations, ROH+2 (protonated alcohols R−O−H)
  • alkyloxonium cations ROH+2 (protonated alcohols)
    • methyloxonium, CH₃OH+2 (protonated methanol)
    • ethyloxonium, CH₃CH₂OH+2 (protonated ethanol)
  • dioxidanonium (hydroxylhydronium), HO−OH+2 (protonated hydrogen peroxide)
• secondary oxonium cations, R₂OH⁺ (protonated ethers R−O−R)
  • dialkyloxonium cations (protonated ethers)
    • dimethyloxonium, (CH₃)₂OH⁺ (protonated dimethyl ether)
• tertiary oxonium cations, R₃O⁺
  • trifluorooxonium, OF+3 (hypothetical)
  • trimethyloxonium, (CH₃)₃O⁺
  • triethyloxonium, (CH₃CH₂)₃O⁺
  • oxatriquinacene, C₉H₉O⁺ (cyclic oxonium ion)
  • oxatriquinane, C₉H₁₅O⁺ (cyclic oxonium ion)
• primary sulfonium cations, RSH+2 (protonated thiols R−S−H)
• secondary sulfonium cations, R₂SH⁺ (protonated thioethers R−S−R)
  • dimethylsulfonium, (CH₃)₂SH⁺ (protonated dimethyl sulfide)
• tertiary sulfonium cations, R₃S⁺
  • trimethylsulfonium, (CH₃)₃S⁺
• tertiary selenonium cations, R₃Se⁺
  • triphenylselenonium, (C₆H₅)₃Se⁺
• tertiary telluronium cations, R₃Te⁺
  • triphenyltelluronium, (C₆H₅)₃Te⁺
• primary fluoronium cations, RFH⁺ (protonated fluorides RF)
• secondary fluoronium cations, R₂F⁺
  • dichlorofluoronium, Cl₂F⁺
• secondary iodonium cations, R₂I⁺
  • diphenyliodonium, (C₆H₅)₂I⁺

• secondary ammonium cations having one double-bonded substitution, R=NH+2
  • diazenium, HN=NH+2 (protonated diazene)
  • guanidinium, C(NH₂)+3 (protonated guanidine) (has a resonance structure and a planar molecular geometry)
• tertiary ammonium cations having one triple-bonded substitution, R≡NH+
  • nitrilium, R−C≡NH⁺ (protonated nitrile)
  • diazonium or diazynium, N≡NH⁺ (protonated nitrogen, in other words, protonated diazyne)
• cyclic tertiary ammonium cations where nitrogen is a member of a ring, RNH⁺R (the ring may be aromatic)
  • pyridinium, C₅H₅NH⁺ (protonated pyridine)
• quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions, R=NR+2
  • iminium, R₂C=NR+2 (substituted protonated imine)
  • diazenium, RN=NR+2 (substituted protonated diazene)
  • thiazolium, [C₃NSR₄]⁺(substituted protonated thiazole)
• quaternary ammonium cations having two double-bonded substitutions, R=N⁺=R
  • nitronium, [NO₂]⁺
  • bis(triphenylphosphine)iminium, ((C₆H₅)₃P=)₂N⁺
• quaternary ammonium cations having one triple-bonded substitution and one single-bonded substitution, R≡NR⁺
  • diazonium, N≡NR⁺ (substituted protonated nitrogen, in other words, substituted protonated diazyne)
  • nitrilium, RC≡NR⁺ (substituted protonated nitrile)
• tertiary oxonium cations having one triple-bonded substitution, R≡O⁺
  • acylium ions, R−C≡O⁺ ↔ R−C⁺=O
  • nitrosonium, N≡O⁺
• cyclic tertiary oxonium cations where oxygen is a member of a ring, RO⁺R (the ring may be aromatic)
  • pyrylium, C₅H₅O⁺
• tertiary sulfonium cations having one triple-bonded substitution, R≡S⁺
  • thionitrosyl, N≡S⁺
• dihydroxyoxoammonium, [H₂NO₃]⁺ (protonated nitric acid)
• trihydroxyoxosulfonium, [H₃SO₄]⁺ (protonated sulfuric acid)

• hydrazinediium or hydrazinium(2+) dication, H₃N⁺−⁺NH₃ (doubly protonated hydrazine, in other words, doubly protonated diazane)
• diazenediium cation, H₂N⁺=⁺NH₂ (doubly protonated diazene)
• diazynediium cation, HN⁺≡⁺NH (doubly protonated dinitrogen, in other words, doubly protonated diazyne)

The extra bond is added to a less-common parent hydride, a carbene analog, typically named -ene or -ylene, which is neutral with 2 fewer bonds than the more-common hydride, typically named -ane or -ine.

• borenium cations, R₂B⁺ (protonated borylenes a.k.a. boranylidenes)
• carbenium cations, R₃C⁺ (protonated carbenes) have a tricoordinated carbon atom with a +1 charge.
  • alkenium cations, C
    _nH⁺
    _2n+1 (n ≥ 2) (protonated alkenes)
    • methenium cation, H₃C⁺ (protonated methylene)
    • ethenium, C₂H+5 (protonated ethene)
  • benzenium, C₆H+7 (protonated benzene)
  • tropylium, C₇H+7 (protonated tropylidene)
• silylium cations, R₃Si⁺ (protonated silylenes)
• nitrenium cations, R₂N⁺ (protonated nitrenes)
• phosphinidenium cations, R₂P⁺ (protonated phosphinidene)
• mercurinium cations, R₃Hg⁺ (protonated organomercury compounds; formed as intermediates in oxymercuration reactions)

• diphenylcarbenium, (C₆H₅)₂CH⁺ (di-substituted methenium)
• triphenylcarbenium, (C₆H₅)₃C⁺ (tri-substituted methenium)

• carbynium ions (protonated carbynes) have a carbon atom with a +1 charge.
  • alkynium cations, C
    _nH⁺
    _2n-1 (n ≥ 2) (protonated alkynes)
    • methynium cation, H₂C⁺ (protonated methylidyne radical)
    • ethynium, C₂H+3 (protonated ethyne)

• Carbonium ion
• Lyonium ion, a protonated solvent molecule
• Lyate ion, a deprotonated solvent molecule

• Ions and Radicals, Queen Mary University of London
• Onium compounds at the U.S. National Library of Medicine Medical Subject Headings (MeSH)