Which of the following particles/rays is emitted in a radioactive decay in which neither the atomic number nor the mass number changes?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Gamma
The atomic number counts the protons and the mass number counts the nucleons. A decay leaves both unchanged only if the emitted radiation carries neither charge nor nucleons, which is true of a gamma ray, a photon given out when an excited nucleus drops to a lower energy level.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Gamma Decay – “There are energy levels in a nucleus, just like there are energy levels in atoms. When a nucleus is in an excited state, it can make a transition to a lower energy state by the emission of electromagnetic radiation. As the energy differences between levels in a nucleus are of the order of MeV, the photons emitted by the nuclei have MeV energies and are called gamma rays.”