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. 

 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.