A small fraction of the alpha particles scattered at angle θ > 90º is due to the following reasons.
(i) If impact parameter ‘b’ reduces to zero, coulomb force increases, and hence alpha particles are scattered at angle θ > 90º, and only one alpha particle is scattered at angle 180º.
Conclusions:
(i) Entire positive charge and most of the mass of the atom is concentrated in the nucleus with the electrons some distance away.
(ii) Size of the nucleus is about 10–15m to 10–14m, while size of the atom 10–10m, so the electrons are at distance 104m to 105m from the nucleus, and being large empty space in the atom, most a particles go through the empty space.