Final answer:
According to Thomson's model, alpha particles should have passed through gold foil unimpeded, but Rutherford observed significant deflections indicating a nucleus.
Step-by-step explanation:
If J.J. Thomson's model of the atom was correct, Ernest Rutherford would have expected to see alpha particles passing straight through the gold foil with perhaps a slight speed reduction, implying that the positive charge and mass of the atoms were uniformly distributed. In Thomson's model, known as the 'plum pudding' model, alpha particles would deflect only slightly if at all, due to the evenly spread positive charge. In Rutherford's gold foil experiment, however, he observed some alpha particles deflecting at large angles and some even bouncing back, indicating the presence of a dense, positively charged nucleus. This led to the formation of Rutherford's planetary model of the atom, where most of the atom's mass and positive charge are concentrated in a central nucleus, surrounded by electrons in motion, like planets orbiting the sun.