If the positive charge were uniformly distributed over the atom, then the electric field in the atom with a uniform distribution of the charge would increase from the periphery of the atom and weak Coulomb forces would act on the positive particle in the atom. In addition, the mass of the nucleus would be distributed over the atom and its density would be low. An alpha particle could then pass through the atom without experiencing much resistance.
An alpha particle can be deflected at large angles only if the charge and mass are concentrated in a very small volume.
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