What evidence is there to suggest that most of the mass of an atom is
concentrated in a small, positively charged nucleus?

Question

What evidence is there to suggest that most of the mass of an atom is 
concentrated in a small, positively charged nucleus?

anonymous · Answer

In 1911 Rutherford decided to test the plum pudding model and to do this he got a sheet of gold foil and he fired positive $$He ^{2+}$$ ions at the gold foil. He expected to see that these particles (ions) would simply pass straight through with only very minor losses of energy and very slight deflections.

However, what he saw were major deflections and in some cases a deflection almost 180 degrees back. My diagram isn't very clear but the lines show positive helium ions being fired towards the gold atom, the ions that went past the nuclear were not deflected by any significant measure. However those that collided with the nucleus appeared to be reflected back at large angles. This led Rutherford to theorize that there was a large dense centre to the atom and that it was positive as it was repelling the positive helium ion and seen as like charges repel. 
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hope that helped!