Muonium is an
exotic atom made up of an
antimuon and an
electron, which was discovered in 1960 and is given the chemical symbol . During the muon's lifetime, muonium can enter into compounds such as muonium chloride () or sodium muonide (). Due to the mass difference between the antimuon and the electron, muonium () is more similar to
atomic hydrogen () than
positronium (). Its
Bohr radius and ionization energy are within 0.5% of
hydrogen,
deuterium, and
tritium.