1. A cation formed by the addition of a proton or other cation to a neutral molecule, especially in organic chemistry.
Usage: chemistry; technical

Examples

• The oxonium ion is a common onium species formed when water accepts a proton.
• In organic synthesis, ammonium salts are onium compounds with nitrogen at the center.
• The formation of an onium intermediate is a key step in many electrophilic addition reactions.
• Sulfonium ions are onium cations containing a positively charged sulfur atom.
• Chemists use onium compounds to understand reaction mechanisms in detail.