The two possible nuclei to complete the reactions given below are, respectively
(1) \( {}^{14}_{7}\mathrm{N} + {}^{4}_{2}\mathrm{He} = {}^{17}_{8}\mathrm{O} + \) ----
(2) \( {}^{16}_{8}\mathrm{O} + {}^{2}_{1}\mathrm{H} = {}^{14}_{7}\mathrm{N} + \) ------
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( {}^{1}_{1}\mathrm{H} \), \( {}^{4}_{2}\mathrm{He} \)
In every nuclear reaction the mass number (upper index) and the atomic number (lower index) are conserved: the totals on the two sides must be equal.
Reaction (1) \( {}^{14}_{7}\mathrm{N} + {}^{4}_{2}\mathrm{He} = {}^{17}_{8}\mathrm{O} + \) ---- — let the missing nucleus be \( {}^{A}_{Z}\mathrm{X} \).
\( 14 + 4 = 17 + A \;\Rightarrow\; A = 1 \)
\( 7 + 2 = 8 + Z \;\Rightarrow\; Z = 1 \)
So it is \( {}^{1}_{1}\mathrm{H} \), a proton.
Reaction (2) \( {}^{16}_{8}\mathrm{O} + {}^{2}_{1}\mathrm{H} = {}^{14}_{7}\mathrm{N} + \) ------ —
\( 16 + 2 = 14 + A \;\Rightarrow\; A = 4 \)
\( 8 + 1 = 7 + Z \;\Rightarrow\; Z = 2 \)
So it is \( {}^{4}_{2}\mathrm{He} \), an alpha particle.
NCERT (2026-27):
Controlled Thermonuclear Fusion – “… In a nuclear reaction, elements may be transmuted. Thus, the number of atoms of each element is not necessarily conserved in a nuclear reaction. However, the number of protons and the number of neutrons are both separately conserved in a nuclear reaction. … In nuclear reactions (e.g., Eq. 13.10), the number of protons and the number of neutrons are the same on the two sides of the equation.”