Assertion (A) The impact parameter for -particles by 180^ is zero. Reason (R) Zero impact parameter means that the -particles tend to hit the centre of the nucleus.

Step-by-step solution

Impact parameter, b= z e^2 2 4 ₀ ( 1 2 m v^2 ) For =180^ , 90^ =0 , impact parameter, b=0 Hence, -particles tend to hit the centre of nucleus. Thus, both A and R are true and R is a correct explanation for A .