From the gold foil experiment Rutherford was able to estimate the closest approach of an alpha to the gold nucleus by equating the initial kinetic energy of the alpha to the electric potential energy of the gold nucleus and alpha system at closet approach.
The experimental data was consistent with the theoretical predictions which were based on the assumption the only force between an alpha particle and a nucleus was the Coulomb repulsive force obeying the inverse square law.
The distance of closest approach gave an upper bound to the radius of the nucleus.
Soon after the gold foil experiment, other experiments were performed with other metal being used as targets and it was found that as the mass of a nucleus decreased scattering angles were more probable than for a target made of gold.
Again the assumption that the only force of interaction was the Coulomb repulsive force was seen to be valid because the experimental data agreed with the theoretic predictions except for large angle deflections.
The scattering angle increases as an alpha particle comes closer to a nucleus.
The departure of the experimental data from the theoretical predictions was thought to be be due to an alpha "touching" a nucleus with the result that there was a second force acting between an alpha and a nucleus.
Noting when experiment and theory did not agree enable an estimate to be made for the radius of a nucleus.
All other things being equal, using Lithium as a target would result in a greater probability of smaller angles of deflection and a greater probability of large angle deflections departing from theoretical predictions, based on the Coulomb force being the only force of interaction between an alpha and a nucleus, because for a given energy of alpha particles an alpha's nearest approach to a lithium nucleus is smaller than that for a gold nucleus.