The exotic atom muonium (Mu) can be considered a light isotope of hydrogen (H), in addition to the well-known heavy isotopes deuterium (D) and tritium (T).

The isotopes of hydrogen:
Hydrogen: one electron, one proton
Deuterium: one electron, one proton, one neutron
Tritium: (radioactive) one electron, one proton, two neutrons

Muon: the first elementary particle discovered that is not found in ordinary atoms. The muonium atom (a positive muon that binds an electron) behaves chemically like the isotopes of hydrogen