If an emission of β− particle takes place from a nucleus, the ratio of number of neutrons to number of protons in the nucleus
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → will decrease
In β− decay a neutron inside the nucleus changes into a proton, emitting an electron and an antineutrino.
\( \displaystyle n \rightarrow p + e^{-} + \bar{\nu} \)
So N becomes \( N - 1 \) and Z becomes \( Z + 1 \): the numerator falls and the denominator rises, so N/Z always decreases, whatever the nucleus. This is why β− decay occurs in neutron-rich nuclei.
Why the other options are incorrect
Option (2) will increase — Incorrect. N/Z increases in β+ decay or electron capture, where a proton turns into a neutron.
Option (3) will remain the same — Incorrect. The ratio would stay the same only if N and Z did not change, but both change in β decay.
Option (4) may increase or decrease depending on the type of nucleus — Incorrect. The change \( N \rightarrow N-1 \), \( Z \rightarrow Z+1 \) is the same for every nucleus, so the direction of change is always 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):
Beta Decay – “… In beta-minus decay, a neutron transforms into a proton within the nucleus according to
\( n \rightarrow p + e^{-} + \bar{\nu} \qquad (13.24) \)
whereas in beta-plus decay, a proton transforms into neutron (inside the nucleus) via
\( p \rightarrow n + e^{+} + \nu \qquad (13.25) \)
These processes show why the mass number \( A \) of a nuclide undergoing beta decay does not change; one of its constituent nucleons simply changes its character according to Eq. (13.24) or (13.25).”