-particles are projected with identical kinetic energies towards thin foils of , , , and . For which — Atomic Structure Chemistry Question

💡 Solution & Explanation

The distance of closest approach ($r_{min}$) is directly proportional to the target's atomic number ($Z$) since $K.E. = \frac{1}{4\pi\epsilon_0} \frac{Z_1 Z_2 e^2}{r_{min}}$. Minimum $Z$ corresponds to $Ca$ ($Z=20$), yielding the minimum $r_{min}$.