The Geiger–Marsden experiment was an experiment, that was carried out by Ernest Rutherford and his assistants Hans Geiger and Ernest Marsden at the University of Manchester between 1908 and 1913. A beam of alpha particles (from the newly discovered element Radium) was shot at a thin gold foil.
The scientists made three observations:
- The majority of the alpha particles went through the gold foil essentially undeflected.
- Some alpha particles were deflected by significant amounts.
- Occasionally some alpha particles were reflected. That is, they bounced back toward the alpha source.
The then-prevailing "plum pudding" model of the atom posited that atoms were essentially "soft", with protons distributed throughout, and electrons running around with them. The alpha particles would not have bounced back under that model. The experiment showed that the positive charge, that is, the protons, must have been concentrated in one location (the nucleus) that was "hard". If the alpha particle got too close to that, the electric force repelling it would be so enormous that the alpha particle could turn around and go back.
So this experiment established the nuclear model of the atom. The atom is almost entirely empty space.