There were once several competing models of the atom.
The nuclear model pictured the atom as being like a mini solar
system, with light electrons orbiting a heavy nucleus, while the plum
pudding model pictured the atom as being a uniformly positively
charged ball dotted with electrons.
One of the most convincing pieces of evidence for the
nuclear model of the atom is produced by the Geiger Marsden
experiment. Positively charge alpha particles are fired at a thin
sheet of gold leaf. Most of the alpha particles pass through through
the gold leaf without noticeable deflection. A few are deflected
through a small angle, but surprisingly, some are deflected through a
very large angle.
A full mathematical analysis shows that the numbers
being deflected at any angle agrees with an inverse square law of
repulsion from the nucleus.
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