Question
Define the distance of closest approach.

Answer

The minimum distance upto which an energetic $\alpha$-particle travelling directly towards a nucleus can move before coming to rest and then retracing its path is known as distance of closest approach.
$F=\frac{1}{4 \pi \in_0} \frac{(2 e)(Ze)}{r^2}$
Where $r$ is the distance of $\alpha$-particle from nucleus.

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