The Rutherford experiment is a 1911 experiment conducted to determine the internal structure of an atom, specifically to determine whether the structure of an atom was homogeneous (uniformly distributed particles, Thomson's proposal) or planetary (smaller particles orbiting a larger central one, Rutherford's proposal). The planetary model was found to be correct.
Thomson's model argued that there were elementary electrons with charge uniformly distributed throughout the atom alongside other particles with positive charges. Rutherford's model argued that the positive charges were concentrated in a single, much larger positive charge carrier called the nucleus. To test the claims, a thin pure gold foil was bombarded with a beam of particles () and their impact points were recorded in an (almost) 360° radius around the foil.
In Thomson's case, the expectation was that the particles would have passed straight through the atom (in this case the gold foil) when shot through it, as the homogeneous distribution would have lead to a net zero deviation on average.
In Rutherford's case, the central nucleus, distributed centrally, would have had a high enough charge to significantly deflect the incoming particles, creating an apparent distortion in the trajectory of the test particles. Most importantly, a test particle that happened to strike the nucleus itself would almost certainly bounce off the nucleus due to gold being much heavier than helium (besides being anchored in a solid foil), thus resulting in trajectories that ended up striking the detector in front of the foil instead of behind. Such a measurement would be impossible in Thomson's model.
The empirical results actually did record backwards hits resulting from helium atoms bouncing back, which consequently proved Rutherford right. It also confirmed his formula for Cross section.