The intermolecular potential energy for the molecules A, B, C and D given below suggests that :

Solution

Chemistry ma’am · 3 steps

Read each well for two things: how deep it goes and where its bottom sits on the distance axis.

1. Reading a well. The bottom of each curve gives the bond length (its x-value) and the bond enthalpy (its depth below zero). A deep, narrow well means the energy rises steeply as soon as the atoms move from their best distance, so that bond is the stiffest.

   Pair	Bond length (minimum)	Well depth	Shape of the well
   A-A	about 68 pm, shortest	about 450 kJ mol⁻¹	narrow
   A-B	about 95 pm	about 550 kJ mol⁻¹, deepest	deep and narrow
   A-C	about 127 pm	about 420 kJ mol⁻¹	wider
   A-D	about 147 pm, longest	about 340 kJ mol⁻¹, shallowest	broad and shallow

2. Check each option. A-D has the longest bond, not the shortest. The largest bond enthalpy belongs to A-B, not A-A. The graph says nothing direct about electronegativity, and the long A-D bond only suggests D is a larger atom.
3. So. A-B has the deepest, steep-walled well, so it is the stiffest bond. The answer is (a).

The idea to keep. In a potential energy curve, the x-value of the minimum is the bond length and its depth is the bond enthalpy. A deep, narrow well is a strong, stiff bond.

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