The trihydrogen cation, also known as protonated molecular hydrogen or H₃⁺, 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 H₃⁺ plays in the gas-phase chemistry of the ISM is unparalleled by any other molecular ion. The cation is also the simplest triatomic molecule, since its two electrons are the only valence electrons in the system. It is also the simplest example of a three-center two-electron bond system.