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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.
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Reason (R) Zero impact parameter means that the $\alpha$-particles tend to hit the centre of the nucleus.
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The correct answer is:
Both $\mathrm{A}$ and $\mathrm{R}$ are true and $\mathrm{R}$ is a correct explanation for $A$.
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 $\mathrm{A}$.
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 $\mathrm{A}$.
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