Rutherford did not expect to see large deflections since the alpha (α)-particles were much heavier than the protons in the gold atoms.
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Rutherford expected the alpha (α)-particles to be deflected by the sub-atomic particles within the gold atoms as they passed through the foil.
Alpha (α)-particles are doubly-charged helium ions consisting of two protons and two neutrons, with a mass of 4 atomic mass units (u).
Rutherford selected gold foil because he wanted the thinnest possible layer to increase the likelihood of interactions between the alpha particles and the atoms in the foil.
Rutherford designed an experiment where fast-moving alpha (α)-particles were directed at a thin gold foil.