A muonic atom has a negative muon bound in the Coulomb potential of an atomic nucleus. Its much larger reduced mass than in an electronic chemical atom greatly reduces the orbital scale, increasing sensitivity of transition energies to the nuclear charge radius and nuclear charge distribution. Measuring muonic X-ray transitions therefore complements electron scattering. Strong relativistic and nuclear-size effects in heavy systems require a treatment beyond a point-nucleus nonrelativistic Coulomb spectrum.
A bound negative muon can be absorbed by a nucleus through the weak charged current, changing a proton into a neutron and emitting a muon neutrino. This competes with ordinary muon decay and changes the observable disappearance lifetime in a muonic atom.
An atom is monovalent in a specified bonding or conduction model when its relevant valence is one. In the simple metallic free electron model, one monovalent atom per primitive unit cell supplies one conduction electron per cell. Counting the two spin states then fills half of a single energy band at zero temperature.
Electrons partially shield an atomic nucleus electric charge. At large distances a neutral chemical atom therefore does not generate the unscreened Coulomb potential of its bare atomic nucleus.
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