Two scientists, Geiger and Mardsen, used alpha particles in a very famous experiment to determine the structure of atoms!
When a beam of alpha particles was aimed at the thin gold foil in a vacuum...
- Most of the alpha particles passed directly through the gold foil.
- Some alpha particles deflected at a small angle.
- A few of the alpha particles deflected at a large angle, back towards the source.
Why is it important that the experiment was done in vacuum?
- To reduce the collisions between air molecules and alpha particles
- To avoid energy loss of alpha particles
- To make sure alpha particles are not deflected by air molecules
- To ensure alpha particles reach the gold foil
Just a quick recap on what an alpha particle is:
Based on the observations from the experiment, scientists concluded that each atom has a small, dense, positively-charged nucleus. How did the scientists come up with this conclusion? 🤔
- Most of the alpha particles directly passing through the gold foil suggests that most of the space of the atom is an empty space.
- Some alpha particles deflecting at a small angle suggests that there is positive charge in the atom. Alpha particles are positively charged, so they will be deflected by the positive charges due to electrostatic repulsion force.
- A few alpha particles coming back towards the source suggests that the positive charges are concentrated as a small mass in the atom. The number of alpha particles deflecting backwards was very small, which tells us that chance of alpha particles directly hitting the positive charge was very small. Therefore, the positive charges should be occupying only a small space of the atom.