Onium Ions

Mononuclear parent onium ions

Parent cation	Name	Parent cation	Name
NH₄⁺	ammonium	SeH₃⁺	selenonium
PH₄⁺	phosphonium	TeH₃⁺	telluronium
AsH₄⁺	arsonium	FH₂⁺	fluoronium
SbH₄⁺	stibonium	ClH₂⁺	chloronium
BiH₄⁺	bismuthonium	BrH₂⁺	bromonium
OH₃⁺	oxonium	IH₂⁺	iodonium
SH₃⁺	sulfonium

Please observe that each of these -onium ions has a positive charge. The reason they have an overall positive charge is that they each contain one bond (e.g., an addition bond to hydrogen when it accepts the H⁺ ion) more than is necessary to form a neutral compound (e.g., H₂0 accepts a H⁺ to become H₃O⁺; HF accepts a H⁺ to become H₂F⁺, etc.). You should also remember that the suffix -onium usually always refers to a positively charged ion.

To name an onium ion, use the parent (root) name of the element in the compound and add -onium to the name. A certain amount of phonetic juggling occurs, but the root name is still recognizable.

If the element in the compound that accepts the additional H⁺ ion is "oxygen" then the onium ion would be called the "oxonium" ion (the common name for oxonium is hydronium, but IUPAC rules state that oxonium is the correct name).
If the element in the compound was sulfur, then the onium ion would be called the "sulfonium" ion.
If the element in the compound was a chlorine, then the onium ion would be called the "chloronium" ion.

Finally, if the "oxygen" that accepts the additional H⁺ ion is part of an alcohol, then the resulting onium ion would be an "alkyloxonium" ion, since there is a carbon group (alkyl group) that would also now be attached to that oxygen.