In a radioactive decay, neither the atomic number nor the mass number changes. Which of the following is emitted in the decay?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Photon
In gamma decay a nucleus left in an excited state drops to a lower energy state and emits the energy difference as a gamma-ray photon. A photon has no charge and no mass number, so \( Z \) and \( A \) stay the same.
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.”