An exotic atom is an atom 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.
- Antiprotonic helium, in which an antiproton pairs with an electron and helium nucleus 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.
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