Rutherford's Scattering Experiment
At the start of the 20th century, Ernest Rutherford devised an experiment to investigate the structure of atoms.
Positively-charged Alpha particles were fired at a very thin piece of gold foil in the apparatus shown below. Because of the vacuum, the Alpha particles were able to travel freely.
Every time an Alpha particle hit the fluorescent screen, the screen glowed for a short time. The microscope was moved all around the outside of the circular fluorescent screen, so that the number of Alpha particles hitting the screen at every position could be observed.
The video below explains the observations Rutherford's team recorded.
Rutherford deduced three main points about atoms from these results:
1. Because most of the positively-charged Alpha particles passed straight through the gold atoms in the foil, most of the atom must be empty space.
2. Because only very, very few positively-charged Alpha particles were deflected backwards in almost the direction they had come from, most of the mass of the atom must be concentrated in a very small central area.
3. Because some of the positively-charged Alpha particles were deflected backwards by the nucleus, the nucleus must be positively-charged.
Because of these findings we can deduce the atom's structure.