Why is Rutherford’s experiment called the gold foil experiment?

The Geiger–Marsden experiments (also called the Rutherford gold foil experiment) were a series of landmark experiments by which scientists discovered that every atom contains a nucleus where its positive charge and most of its mass is concentrated. They deduced this by observing how alpha particles are scattered when they strike a thin metal foil. The experiment was performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.

What they found, to great surprise, was that while most of the alpha particles passed straight through the foil, a small percentage of them were deflected at very large angles and some were even backscattered. Because alpha particles have about 8000 times the mass of an electron and impacted the foil at very high velocities, it was clear that very strong forces were necessary to deflect and backscatter these particles.

Rutherford explained this phenomenon with a revitalized model of the atom in which most of the mass was concentrated into a compact nucleus (holding all of the positive charge), with electrons occupying the bulk of the atom's space and orbiting the nucleus at a distance.

With the atom being composed largely of empty space, it was then very easy to construct a scenario where most of the alpha particles passed through the foil, and only the ones that encountered a direct collision with a gold nucleus were deflected or scattered backwards.

Good description of the experiment with applet can be found here: http://micro.magnet.fsu.edu/electromag/java/rutherford/