What is Helonium? Credit: Mondolithic studios Helonium or Helium hydride is a molecular ion consisting of a helium atom and a hydrogen ion (HeH + ) and it is considered to be the first molecule of the universe. It is viewed as a protonated helium and it is the lightest heteronuclear ion. Discovery Helonium was first produced in the laboratory in Berkeley in 1925, under vacuum conditions with the aid of electricity by injecting protons of known energy into a rarefied mixture of hydrogen and helium. Although it is stable in isolation, it cannot be prepared in bulk because it is extremely reactive. Properties It is the strongest known acid with pKa -63 (for comparison hydrochloric acid pKa is -6.3) It is isoelectronic with molecular hydrogen H2 Its covalent bond is dative with a length of 0.772 A 80% of the electron charge is closer to the helium nucleus than to the hydrogen nucleus Unlike the helium hydride ion, the neutral helium hydride molecule HeH is not stable in the ground state De