JEE Main · 2015 · 04 Apr 2015 · Offline · Chemical Bonding and Molecular Structure

Rank these interactions by how steeply they weaken with distance. An ion sets up a field that falls as $1/r^2$, and a dipole placed in it responds to how fast that field changes, so differentiating brings in another factor of $1/r$ and the ion-dipole interaction scales as $1/r^3$. For comparison the ion-ion interaction fades only as $1/r$, while London forces die away far faster, around $1/r^6$. The $1/r^3$ dependence is the one belonging to the ion-dipole interaction.