In the alpha scattering experiment performed by Rutherford, alpha particles were bombarded on gold foil. Most of the alpha particles did not get deflected indicating most of the space in atom is empty. Some of the alpha particles got scattered or deflected from the original path, indicating presence of positive charge particles (proton).
Due to the small size of magnesium, the scattering of alpha particles will get reduced in the Rutherford experiment. Also, due to large number of protons in gold, the interaction with alpha particles will be stronger and Mg with fewer number of protons will have less interaction.