From the plot it is clear that most of the α-particles passed through the foil. Only 0.14% of the incident α particles scatter by more than 1% and about 1 in 8000 deflect by more than 90° α-particles deflected backward due to strong repulsive force. This force will come from positive charge concentrated at the centre as most of the particles get deflected by small angles.
The α-particles trajectory depends on collision's impact parameter (b) for a given beam of α-particles, distribution of impact parameters as beam gets scattered in different directions with different probabilities.
The figure above shows α-particle close to the nucleus suffers large scattering. Impact parameter is minimum for head-on collision α-particles rebound by 180°.
The impact parameter is high, for undeviated α-particles. With deflection angle ≈ 0°.
As these of nucleus was 10–14 m to 10–15 m w.r.t. 10–10 m in size of an atom which is 10,000 to 100,000 times larger. Hence most of the space is empty, only a small percentage of the incident particles rebound back indicates that the number of α-particle goes head-on collision. Hence most of the mass of the atom is concentrated in a small volume.
Thus, Rutherford scattering is a strong tool to determine the upper limit to the size of the nucleus.