The trihydrogen cation or protonated molecular hydrogen (IUPAC name: hydrogenonium ion) is a cation (positive ion) with formula H+3, consisting of three hydrogen nuclei (protons) sharing two electrons.
The trihydrogen cation is one of the most abundant ions in the universe. It is stable in the interstellar medium (ISM) due to the low temperature and low density of interstellar space. The role that H+3 plays in the gas-phase chemistry of the ISM is unparalleled by any other polyatomic ion.