[particles at work] gold foil experiment

[ BBC Bitesize 'Atomic Structure ⇢ Models of the Atom' page 2 ]

Gold Foil Experiment (Rutherford)

In 1905, Ernest Rutherford did an experiment to test the plum pudding model. His two students, Hans Geiger and Ernest Marsden, directed a beam of alpha particles at a very thin gold leaf suspended in a vacuum.

Vacuum

An empty space with no particles and no pressure.

Alpha Particles

Subatomic particles comprising 2 protons and 2 neutrons (the same as a helium nucleus).

Beta Particles

Electrons emitted at high speeds from unstable (radioactive) nuclei.

Usage of Gold Foil:

Only metal that could be rolled out to be very, very thin without cracking.

Most alpha particles went straight through the gold foil, meaning...

most of an atom is empty space.

Some alpha particles were slightly deflected when they hit the gold foil, meaning...

there must be a positively charged region (nucleus) in an atom. Like charges repel, so the positive alpha particles were being repelled by positive charges

A very small number (1 in 8,000) of alpha particles came straight back one they hi the gold foil, meaning...

atoms have a small, concentrated region that is positively charged (nucleus). The tiny number of alpha particles doing this means the chance of being on that exact collision course was very small, and so the 'target' (nucleus) being aimed at had to be equally tiny.