Before Rutherford's experiments: The atom was considered using the Thomson's Plum Pudding Model - a sphere of positive charge with electrons distributed randomly in it
Rutherford's experiments: Involved bombarding gold foil with positively charged particles known as alpha particles
What the atomic model at the time would have predicted: Most of the alpha particles would be deflected straight back
What actually happened: Most of the alpha particles passed straight through, some were slightly deflected and a very small number bounced straight back
Rutherford's conclusions: The atom must be mostly empty space, with a positive central nucleus that contains most of the mass
This is sometimes known as the Geiger and Marsden experiment as it was carried out by two of Rutherford's assistant with those names.