An $\alpha$ -particle of energy ${1\over2} mv^2$ bombards by a heavy nuclear target ofcharge ze.Then the distance of closet approach for the alpha nucleus will be Proportional to
at distance of closest approach K.E = P.E ${1\over 2}mv^2 = {1 \over 4 \pi \epsilon_o} {(ze) (ze) \over ro} \Rightarrow ro = {ze^2 \over \pi epsilon_o mv^2 }$