Carbonium ion

In chemistry, carbonium ion is any cation that has a pentavalent carbon atom, The name carbonium may also be used for the simplest member of the class, properly called methanium (CH+
5), where the five valences are filled with hydrogen atoms.

The next simplest carbonium ions after methanium have two carbons. Ethanium or protonated acetylene C
2H+
3 and ethenium C
2H+
5 are usually classified in other families. The ethanium ion C
2H+
7 has been studied as an extremely rarefied gas by infrared spectroscopy.