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Exotic atom

An exotic atom is an atom, which is to say a system of particles held together primarily by the force due to electric charge, containing one or more unusual constituents. Electrons may be replaced by other negative particles, e.g. a muon or a pi-minus, or the positively charged nucleus can be replaced by other positively charged elementary particles.

Examples include:

• Pionium, consisting of a pi-plus and a pi-minus meson
• Mesonic atom, which is an ordinary atomic nucleus with one or more of its electrons replaced by a negatively-charged meson.
• Positronium, in which a positron pairs with an electron in a semi-stable arrangement.
• Protonium, in which a proton pairs with an antiproton in a semi-stable arrangement.
• An exciton, which is a combination of an electron and a hole in a semiconductor or insulator in an excited state.
• Muonium, consisting of a positive muon and an electron.
• Strange matter, a hypothetical form of ultra-dense matter in which a large number of strange quarks are bound together with an electron "atmosphere" around them.

Compare with:

• Antimatter