Onium ion

Key takeaways

Onium ion belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Onium ion to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Onium ion from memory before moving on to harder problems.

Reference excerpt

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. 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. 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.

Periodic table

Onium ions by group

Group 13 (boron group) onium cations boronium cation, BH+4 (protonated borane) further boronium cations, BxH+y (protonated boranes)

Group 14 (carbon group) onium cations Carbonium ions (protonated hydrocarbons) have a pentacoordinated carbon atom with a +1 charge. The specific cation CH+5 is called methanium.. Typically named for the parent hydrocarbon, e.g. C2H+7 is ethanium. silanium (sometimes silonium), SiH+5 (protonated silane) (should not be called siliconium) disilanium, Si2H+7 (protonated disilane) further silanium cations, SinH+2n+3 (protonated silanes) germonium, GeH+5 (protonated germane) Unstable derivative known of R3Ge+. stannonium, SnH+3 (protonated stannylene, SnH2) (not protonated stannane SnH4) stable at cryogenic conditions.

Group 15 (pnictogen) onium cations ammonium (IUPAC name azanium), NH+4 (protonated ammonia (IUPAC name azane)) exists in aqueous solution and as salts. primary, secondary, and tertiary organic derivatives [NHnR4−n]+, derived from protonation of amines; e.g. methylammonium halides, diethylammonium chloride, trimethylammonium quaternary ammonium cations NR+4; e.g. tetrabutylammonium phosphonium, PH+4 (protonated phosphine) primary, secondary, and tertiary organic derivatives [PHnR4−n]+, derived from protonation of phosphines quaternary phosphonium cations PR+4; e.g. tetraphenylphosphonium arsonium, AsH+4 (protonated arsine) known as derivatives or as an unstable salt at cryogenic temperatures. stibonium, SbH+4 (protonated stibine) known as derivatives but no salts are known. bismuthonium, BiH+4 (protonated bismuthine) only known as derivatives.

Group 16 (chalcogen) onium cations 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.) Organic derivatives can be primary (ROH+2, protonated alcohols), secondary (R2OH+, protonated ethers), or ternary (R3O+, as trimethyloxonium). peroxonium, H3O+2 (protonated hydrogen peroxide) sulfonium, H3S+ (protonated hydrogen sulfide) Organic derivatives can be primary (H2SR+, protonated thiols), secondary (HSR+2, protonated thioethers), or ternary (SR+3, e.g. trimethylsulfonium) selenonium, H3Se+ (protonated hydrogen selenide) Tertiary organic derivatives R3Se+ are known, with trimethylselenonium iodide being the first. telluronium, H3Te+ (protonated hydrogen telluride)

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

Group 17 (halogen) onium cations, halonium ions, H2X+ (protonated hydrogen halides) fluoronium, H2F+ (protonated hydrogen fluoride) chloronium, H2Cl+ (protonated hydrogen chloride) bromonium, H2Br+ (protonated hydrogen bromide) iodonium, H2I+ (protonated hydrogen iodide)

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

Group 18 (noble gas) onium cations hydrohelium or helonium, better known as helium hydride ion, HeH+ (protonated helium) known in gas phase. neonium, NeH+ (protonated neon) known in gas phase. argonium, ArH+ (protonated argon) known in gas phase. kryptonium, KrH+ (protonated krypton) known in gas phase. xenonium, XeH+ (protonated xenon) known in gas phase.

Onium cations with monovalent substitutions 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+

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Worked examples

Example 1 — a first encounter with Onium ion

Start with the simplest possible case. Write down what Onium ion claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Onium ion before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Onium ion ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Onium ion

In research: Onium ion appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Onium ion in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Onium ion is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cations, Chemical nomenclature, Set index articles, so understanding it makes those chapters shorter.
In everyday life: Look for Onium ion outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Onium ion in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Onium ion means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
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Frequently asked questions

What is Onium ion in simple terms?

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 deriv…

Why does Onium ion matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Onium ion?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Onium ion.