Common
Exotic atom
Atoms composed of exotic particles
Exotic atom
An exotic atom is an otherwise normal atom in which one or more subatomic particles have been replaced by other particles. 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 and no exotic atom thus far observed can persist under normal conditions.
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