Quirky Chemistry: What is Muonium?

There are some 90 natural-occurring elements on Earth. There are more than a dozen others as well. Muonium is not one of these. Of these more than one-hundred elements found in the periodic table of the elements, all consist of atoms made up of negative particles called electrons traveling in orbits about respective nuclei containing one or more positively charged protons. Almost all of these have neutrons within their nuclei as well. Though it has an electron traveling in its orbital, muonium possesses no proton. Instead, its nucleus consists of an antimuon. Since the antimuon has a half-life of only 2.2 microseconds (2.2 ×10^-6 seconds), one must act quickly to study this elusive "element." Interestingly, enough information has been gathered to allow publication of a book entitled Muon and Muonium Chemistry.¹

Discussion

Muonium, chemical symbol Mu, may be compared to ordinary hydrogen, since hydrogen isotope 1 has only one particle in its nucleus and a lone 1s electron traveling about it. Muonium, too, contains its antimuon as the lone nuclear particle and has a 1s electron moving about that. Muonium is considerably lighter than hydrogen, the lightest of the elements, since it weighs a mere 200× the mass of an electron, compared to the proton, which weighs more than 1800× what an electron weighs. One could thus envision (if only time allowed) the muonium equivalent of a hydrogen balloon that could lift much greater loads into the atmosphere with little effort.

Where Do Antimuons Come From?

Keeping in mind that an antimuon is not the same thing as a muon, where do antimuons come from? Cosmic rays, which consists (among others things) of naked atoms - that is their nuclei (especially protons of the super-abundant hydrogen gas) - collide at terrifically high energies, leading (through a series of preliminary decays) to the formation of a muon and antimuon pair. A more viable production method is to collide a positron with an electron.

Compounds Formed and Studied

Hydrated ions, such as the hexahydrate Mu(H₂O)₆⁺ plus one ion, and compounds such as the chloride have been both prepared and studied. Certain quantum mechanical properties involving the spin of the antimuon, which are beyond the scope of this article, have made such chemical studies possible, despite the relatively short period of existence of muonium products.

Ongoing Research

Interestingly, a second "element," muonium helium is now undergoing investigation, and is the object of careful research.² Note: Unfortunately, the muonium discussed in this article is sometimes contrasted with another substance that has been called, true muonium. The two should not be confused.

¹ Muon and Muonium Chemistry, by Daniel C. Walker, Cambridge University Press, 1983.
² See, for instance, Science, "Kinetic Isotope Effects for the Reactions of Muonic Helium and Muonium with H2," Fleming, et. al., January 28, 2011.