Geiger and Marsden had observed alpha particles
bouncing straight back from a sheet of gold foil.
force that could cause such large deflections had to
be large. The positively charged alpha particles were
being repelled by a massive positively charged object.
positive charge in an atom of gold was known, but if
this charge were collected on a sphere the size of the
atom, the repulsion would be far too weak. To explain
the force experienced by the alpha particle the charge
would have to be collected in a much smaller sphere.