When an α -particle of mass   m moving with a undefined velocity bombards on a heavy nucleus of charge Ze , its distance of the closest approach from the nucleus depends on m as:

Step-by-step solution

At the distance of the lowest approach, total K.E. of α -particle changes to P.E. So, 1 2 m v 2 = K Q . q r = K Z e 2 e r r = 4 K   Z e 2 m v 2 ⇒ r ∝ 1 m r ∝ 1 m