When radon-220 nucleus emits a gamma ray, what nucleus remains?

Gamma ray is an energy emission. In Gamma decay there is no any particle decay from the parent nucleus. It is just an unstable nucleus becomes as stable by releasing energy as photon. That energy releases as a gamma ray. Hence, there is no change in atomic number or mass number of the daughter atom.

Hence, radon (Rn) remains as same after undergoing a gamma decay but in less energy.

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answer carbon and three oxygen atoms;