(a) (i) Difference between "distance of closest approach" and "impact parameter":

(ii) Distance of closest approach =r=‌

Qq

4πε0E

‌Q=79e=79×1.6×10−19C
‌q=2e=2×1.6×10−19C
‌E=3.95‌MeV
‌‌=3.95×106×1.6×10−19J
‌∴r=(9×109) ‌

(79×1.6×10−19)×(2×1.6×10−19)

3.95×106×1.6×10−19

‌‌=576×10−16m
‌‌=5.76×10−14m
OR
(b) (i) Postulates of Bohr's theory:

Postulate 1: In an atom electrons are revolving around the nucleus in definite circular orbits. These orbits are called 'stationary orbits' and each orbit or shell possesses fixed energy. While revolving in these orbits electrons do not emit any radiation.

Postulate 2: Electrons can move only those permissible orbits where the angular momenta of electrons are integral multiples of ‌

h

2π

where h is the
Planck's constant.
Postulate 3: Transition of electrons may occur from one stationary orbit to another. During such transition energy may be emitted or absorbed following the relation Ei−Ef=hv (where Ei∼Ef is the difference of energies of the two stable orbits).
(ii) Angular momentum of electron revolving in 2‌nd ‌ orbit in Bohr's Hydrogen atom:
Angular momentum =‌

nh

2π

Here, n=2,
∴ Angular momentum =2×‌

h

2π

=‌

h

π