Study of radioactive decay of unstable nuclei resulting in emission of a particle lead us to four different observations listed below :
(A) no change in atomic number and mass number.
(B) no change in mass number but the atomic number decreases by 1.
(C) no change in mass number but the atomic number increases by 1.
(D) atomic number decreases by 2 and the mass number decreases by 4.
Arrange the particles emitted in the above four cases in correct sequence.
Choose the correct answer from the options given below.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → photon, positron, electron, alpha particle.
Each emission changes the atomic number \( Z \) and the mass number \( A \) by the charge and the nucleon number carried away.
(A) No change in \( Z \) or \( A \): the particle has no charge and no mass number, so it is a photon (gamma decay).
(B) \( A \) unchanged, \( Z \) falls by 1: a proton turns into a neutron and a positron is emitted (\( \beta^{+} \) decay).
\( \displaystyle p \rightarrow n + e^{+} + \nu \)
(C) \( A \) unchanged, \( Z \) rises by 1: a neutron turns into a proton and an electron is emitted (\( \beta^{-} \) decay).
\( \displaystyle n \rightarrow p + e^{-} + \bar{\nu} \)
(D) \( Z \) falls by 2 and \( A \) by 4: the nucleus loses \( {}^{4}_{2}\mathrm{He} \), an alpha particle.
The sequence is photon, positron, electron, alpha particle.
NCERT (2026-27):
Radioactivity – “… Three types of radioactive decay occur in nature : (i) α-decay in which a helium nucleus \( {}^{4}_{2}\mathrm{He} \) is emitted; (ii) β-decay in which electrons or positrons (particles with the same mass as electrons, but with a charge exactly opposite to that of electron) are emitted; (iii) γ-decay in which high energy (hundreds of keV or more) photons are emitted.”