(a) Differentiate between 'impact parameter' and 'distance of closest approach' in alpha particle scattering experiment. Identify, situations when…

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Impact parameter is the perpendicular distance of the initial velocity vector of the \(\displaystyle \alpha\) - particle from the centre of the nucleus.

Distance of closest approach: It is the minimum distance between the projected \(\displaystyle \alpha\) - particle and the nucleus of target atom at which the kinetic energy of the \(\displaystyle \alpha\) - particle becomes equal to potential energy of \(\displaystyle \alpha\) - particle in the field of nucleus.

Minimum: When \(\displaystyle \alpha\) - particle rebounds back \(\displaystyle (\theta=\pi)\)

Very large: when \(\displaystyle \alpha\) - particle goes nearly undeviated and has a small deflection \(\displaystyle \left(\theta=0^{\circ}\right)\)

According to Bohr's second postulate electron revolves only in those orbit for which angular momentum is \(\displaystyle \frac{\mathrm{nh}}{2 \pi}\). The observed spectral lines of hydrogen atom confirms this.