Read the information given below to answer
In \( \alpha \)-decay, the mass number of daughter nucleus is four less than that of decaying nucleus (parent nucleus). While atomic number decreases by two. In general, \( \alpha \)-decay of parent nucleus \( {}^{A}_{Z}X \) results in daughter nucleus \( {}^{A-4}_{Z-2}Y \)
\( {}^{A}_{Z}X \longrightarrow {}^{A-4}_{Z-2}Y + {}^{4}_{2}\mathrm{He} \)
From Einstein’s mass energy equivalence relation and energy conservation, it is clear that this spontaneous decay is possible only when mass of decay products is less than the mass of the parent nucleus. By referring the table of nuclear masses, one can check that the total mass of daughter nucleus and \( \alpha \)-particle is indeed less than that of parent nucleus.
The disintegration energy or the \( Q \) value can be calculated by formula \( Q = \Delta m \cdot c^{2} \)
In \( \alpha \)-decay, the mass number of the daughter nucleus is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Four less than that of parent nucleus
The emitted \( \alpha \)-particle, \( {}^{4}_{2}\mathrm{He} \), carries away 4 nucleons (2 protons and 2 neutrons). By conservation of mass number,
\( A = (A - 4) + 4 \)
so the daughter nucleus has a mass number four less than the parent, while its atomic number is two less.
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):
Alpha Decay – “… In this process, it is observed that since 42He contains two protons and two neutrons, the mass number and the atomic number of the daughter nucleus decreases by four and two, respectively.”