The positive muons usually adopt interstitial sites of the crystallographic lattice. In most metallic samples the muon's positive charge is collectively screened by a cloud of conduction electrons. Thus, in metals, the muon is in a so-called diamagnetic state and behave like a free muon. In insulators or semiconductors a collective screening cannot take place and the muon will usually pick-up one electron and form a so-called muonium (Mu=μ++e−), which has similar size (Bohr radius), reduced-mass and ionization energy to the hydrogen atom.