For scattering by an inverse-square field (such as that produced by a charged nucleus in Rutherford's model) the relation between impact parameter b and the scattering angle `theta` is given by, the scattering angle for `b=0` is

Explanation

Given relation: `b=(Ze^(2)cot theta//2)/(4pi epsilon_(0)((1)/(2)mv^(2)))`
For b = 0, `0=(Ze^(2)cot theta//2)/(4pi epsilon_(0)((1)/(2)mv^(2)))` or
`cot""(theta)/(2)=0 :. (theta)/(2)=90^(@)` or `theta=180^(@)`
Thus the scattering angle is `180^(@)` when impact parameter is zero.