Assertion (A) : The force of repulsion between atomic nucleus and a-particle varies with distance according to inverse square law. 

Reason(R) : Rutherford did alpha-particle scattering experiment.  

Both A and R are correct and R is NOT a correct explanation of A

Rutherford confirmed the repulsive force on alpha-particle due to nucleus varies with distance according to inverse square law and that the positive charges are concentrated at the centre and not distributed throughout the atom.