Exotic atom

An exotic atom is an atom consisting of unusual constituents. The electron may be replaced by another negative particle, e.g. a muon a pi-minus, or the positively charged nucleus can be replaced by other positively charged elementary particles.

An example is the pionium consisting of a pi-plus and a pi-minus meson. Another example is positronium, in which a positron pairs with an electron in a semi-stable arrangement.