During Rutherford's experiment, generally, the thin foil of heavy atoms like gold was used and bombarded by α-particles. If the thin foil of light atoms like aluminium etc. is used, what difference would have been observed?

The nucleus of heavy elements like gold has a large positive charge and the nucleus of light elements like aluminium have a small positive charge. The repulsion between alpha particles and heavy nuclei is much larger than the repulsion between alpha particles and light nuclei. So if a lighter element like aluminium is used instead of gold, the number of alpha particles deflected will be much less in comparison to the deflections observed when gold was used.