Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : Isotopes of hydrogen have almost same chemical properties, but difference in their rates of reaction.
Reason R : Isotopes of hydrogen have different enthalpy of bond dissociation.
In the light of the above statements, choose the most appropriate answer from the options given below:

Solution

Chemistry ma’am · 3 steps

Same electrons give same reactions; a heavier atom gives a stronger bond, and a stronger bond reacts slower.

1. Why chemistry matches. Protium, deuterium and tritium all have one proton and one electron, so their electronic configuration is the same, 1s¹. Chemical properties come from electrons, so the isotopes react in the same way and form the same kinds of compounds.
2. Where they differ. The isotopes differ only in neutrons, so in mass. A heavier atom vibrates more slowly in a bond and sits lower in energy. More energy is then needed to break the bond, so the bond dissociation enthalpy rises from H₂ to D₂ to T₂.

   Molecule	Bond dissociation enthalpy	Rate of reaction
   H₂	435.9 kJ mol⁻¹	fastest
   D₂	443.4 kJ mol⁻¹	slower
   T₂	446.9 kJ mol⁻¹	slowest

3. So. A is correct: same reactions, different rates. R is correct, and it explains A, because a stronger bond takes more energy to break and so reacts more slowly. The answer is (d).

The idea to keep. Isotopes share chemistry because they share electrons. Any difference you see, in rate or bond strength, traces back to mass. Heavier isotope means a stronger bond and a slower reaction.

Answer from the NTA final answer key. Solution by PrepWiser.