A species in which some other charged particle replaces the electron or nucleon. An example is an atom in which an electron has been replaced by another negatively charged particle, such as a muon or meson. In this case the negative particle eventually collides with the nucleus with the emission of X-ray photons. Another system is one in which the nucleus of an atom has been replaced by a positively charged meson. An association of an electron and a positron is also regarded as an exotic atom (known as positronium). Positronium has a very short life, before decaying into photons.