what is helonium

Helonium, also known as the helium hydride ion (HeH⁺), is a positively charged ion (cation) composed of a helium atom bonded to a hydrogen atom with one electron removed. It is considered the lightest heteronuclear ion and is believed to be the first molecular compound formed in the Universe shortly after the Big Bang. Helonium is notable for being the strongest known acid and is highly reactive, so it cannot be prepared in bulk form. It was first produced in the laboratory in 1925 and was detected in space for the first time in April 2019 using the SOFIA airborne observatory. This discovery is significant for understanding the early chemistry of the Universe, as helium atoms combined with protons to form HeH⁺, marking the beginning of molecular formation after the Big Bang.