Hydrogen atom (non relativistic) hydrogen atom was solved for the first time within Feynman's path integral formulation of quantum mechanics. This work greatly extended the range of applicability of Feynman's method. Hydrogen atom A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic hydrogen constitutes about 75% of the baryonic mass of the universe. In everyday life on Earth, isolated hydrogen atoms (called "atomic hydrogen") are extremely rare. Instead, hydrogen tends to combine with other atoms
Exotic atom Exotic atom An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) or pions (pionic atoms). Because these substitute particles are usually unstable, exotic atoms typically have very short lifetimes. In a "muonic atom" (previously called a "mu-mesic" atom, now known to be a misnomer as muons are not mesons), an electron is replaced by a muon, which, like the electron, is a lepton. Since leptons are only sensitive