Answer: Nucleus
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Rutherford is most famous for his gold foil experiment, where he shot alpha particles into a piece of gold foil. He saw that there were some random deflections of the alpha particles. If the plum and pudding model (equal distribution of negative charges in ball of positive charge) of that time were true, then there would be no deflection. However, the deflections caused Rutherford to conclude there must be a mass in the atom that was causing the deflections, called the nucleus.