In 1909, Geiger and Marsden, working with Rutherford, fired a narrow beam of alpha particles at a very thin gold foil. The apparatus was in a vacuum, so the alpha particles were not absorbed by air. A detector (a zinc sulfide screen that flashes when hit, viewed through a microscope) was moved around the foil to count the alpha particles scattered at each angle.
• Almost all the alpha particles passed straight through or were deflected by only small angles. So the atom is mostly empty space.
• A few were deflected through large angles, and a very small fraction were deflected by more than 90°, bouncing back towards the source. Only a strong repulsive force from a concentrated positive charge could do this.
• So the positive charge and almost all the mass of the atom are concentrated in a tiny, dense nucleus.This nuclear model replaced the earlier plum pudding model, in which positive charge was spread throughout the atom. Positive charge spread out like that could never produce large-angle deflections. The results showed the nucleus has a radius of about 10-15 m, roughly 100 000 times smaller than the atom (about 10-10 m), with electrons outside it.The foil had to be very thin so that each alpha particle was scattered by at most one nucleus.
• Almost all the alpha particles passed straight through or were deflected by only small angles. So the atom is mostly empty space.
• A few were deflected through large angles, and a very small fraction were deflected by more than 90°, bouncing back towards the source. Only a strong repulsive force from a concentrated positive charge could do this.
• So the positive charge and almost all the mass of the atom are concentrated in a tiny, dense nucleus.This nuclear model replaced the earlier plum pudding model, in which positive charge was spread throughout the atom. Positive charge spread out like that could never produce large-angle deflections. The results showed the nucleus has a radius of about 10-15 m, roughly 100 000 times smaller than the atom (about 10-10 m), with electrons outside it.The foil had to be very thin so that each alpha particle was scattered by at most one nucleus.