Muonium (/ ˈ m juː oʊ n i ə m /) is an exotic atom made up of an antimuon and an electron, which was discovered in 1960 by Vernon W. Hughes and is given the chemical symbol Mu. During the muon's 2.2 µs lifetime, muonium can undergo chemical reactions. Because, like a proton, the antimuon's mass is vastly larger than that of the electron, muonium (μ e) is more similar to atomic hydrogen (p e) than positronium (e e). Its Bohr radius and ionization energy are within 0.5% of hydrogen, deuterium, and tritium, and thus it can usefully be considered as an exotic light isotope of hydrogen.