When a gamma ray is emitted from a nucleus the mass (nucleon) number and the proton number do not change. why?

answer:

in gamma decay, depicted in fig. 3-6, a nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons). the number of protons (and neutrons) in the nucleus does not change in this process, so the parent and daughter atoms are the same chemical element.

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