Assertion (A) The impact parameter for $\alpha$-particles by $180^{\circ}$ is zero. Reason (R) Zero impact…
Assertion (A) The impact parameter for $\alpha$-particles by $180^{\circ}$ is zero.
Reason (R) Zero impact parameter means that the $\alpha$-particles tend to hit the centre of the nucleus.
Both $\mathrm{A}$ and $\mathrm{R}$ are true and $\mathrm{R}$ is a correct explanation for $A$.
Both $\mathrm{A}$ and $\mathrm{R}$ are true but $\mathrm{R}$ is not a correct explanation for $\mathrm{A}$.
$\mathrm{A}$ is true but $\mathrm{R}$ is false.
$\mathrm{A}$ is false but $\mathrm{R}$ is true.
Solution
Impact parameter, $b=\frac{z e^2 \cot \frac{\theta}{2}}{4 \pi \varepsilon_0\left(\frac{1}{2} m v^2\right)}$
For $\theta=180^{\circ}, \cot 90^{\circ}=0$, impact parameter, $b=0$ Hence, $\alpha$-particles tend to hit the centre of nucleus.
Thus, both $\mathrm{A}$ and $\mathrm{R}$ are true and $\mathrm{R}$ is a correct explanation for $\mathrm{A}$.