Arrange the following nuclei in increasing order of the number of neu…

The correct answer is Option (3) → (A), (C), (D), (B)

The number of neutrons is N = A − Z. Hydrogen, deuterium and tritium are the three isotopes of hydrogen (Z = 1), and lithium has Z = 3.

    \( \text{(A) Hydrogen } {}^{1}_{1}\mathrm{H}: \ N = 0 \)
    \( \text{(C) Deuterium } {}^{2}_{1}\mathrm{H}: \ N = 1 \)
    \( \text{(D) Tritium } {}^{3}_{1}\mathrm{H}: \ N = 2 \)
    \( \text{(B) Lithium } {}^{7}_{3}\mathrm{Li}: \ N = 4 \ (3 \text{ in } {}^{6}_{3}\mathrm{Li}) \)

Increasing order: (A), (C), (D), (B).

NCERT (2026-27):
Atomic Masses and Composition of Nucleus – “… Even the lightest element, hydrogen has three isotopes having masses 1.0078 u, 2.0141 u, and 3.0160 u.”

Atomic Masses and Composition of Nucleus – “… The other two isotopes of hydrogen are called deuterium and tritium.”

Discovery of Neutron – “Since the nuclei of deuterium and tritium are isotopes of hydrogen, they must contain only one proton each. … The amount of neutral matter present in the nuclei of these isotopes, expressed in units of mass of a proton, is approximately equal to one and two, respectively.”