Name the particle 'X' emitted in the nuclear reaction given below - ^…

The correct answer is Option (3) → Positron

In every nuclear reaction the mass number and the charge (atomic number) are conserved. Let X have mass number \( A \) and charge number \( Z \); the neutrino \( \nu \) has neither.

    \( \text{Mass number: } 22 = 22 + 0 + A \;\Rightarrow\; A = 0 \)
    \( \text{Charge: } 11 = 10 + 0 + Z \;\Rightarrow\; Z = +1 \)

A particle with no mass number and a charge of \( +1 \) is the positron \( e^{+} \). The reaction is \( \beta^{+} \) decay, in which a proton of the nucleus changes into a neutron, a positron and a neutrino.

Option (1) Proton — Incorrect. A proton has mass number 1, so the mass number would not balance.

Option (2) Electron — Incorrect. An electron has charge \( -1 \); emitting it would need the atomic number to rise to 12 (\( \beta^{-} \) decay), not fall to 10.

Option (3) Positron — Correct. The positron (\( A = 0,\ Z = +1 \)) balances both the mass number and the charge.

Option (4) Neutrino — Incorrect. The neutrino is already shown as \( \nu \); being uncharged, a second one cannot balance the charge.

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 – “A nucleus that decays spontaneously by emitting an electron or a positron is said to undergo beta decay.”

Beta Decay – “… In beta plus (\( \beta^{+} \)) decay, a positron is emitted by the nucleus, as in the decay of

    \( \displaystyle {}^{22}_{11}\mathrm{Na} \rightarrow {}^{22}_{10}\mathrm{Ne} + e^{+} + \nu \qquad (T_{1/2} = 2.6\ \mathrm{y}) \qquad (13.23) \)”