Muonium
Muonium is an exotic atom made up of an antimuon and an electron, which was discovered in 1960 and is given the chemical symbol
Mu
. During the muon's 2.2 µs lifetime, muonium can enter into compounds such as muonium chloride (
Mu
Cl
) or sodium muonide (
Na
Mu
). Due to the mass difference between the antimuon and 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.
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