Rutherford’s experiment

Rutherford’s gold leaf experiment was set up to confirm the widely held model of atomic structure called “The Plum Pudding Model”. In his experiment alpha particles were directed at a very thin gold leaf held in the centre of a detecting strip within a vacuum.

The lead box

The lead box produced a fine beam of particles from the alpha source

 

The Plum Pudding model

In the plum pudding model, so called because the atom was thought to have negative charges(electrons) embedded in a lump of positive charge, rather like currants in a cake.

In the plum pudding model it was expected that all the alpha particles would pass straight through.

Expected results

Rutherford’s results

In the actual experiment a few of the particles were deflected a lot and even rebounded so a different explanation was needed.

Rutherford's experiment results

Rutherford explained the results by a new model in which the atom was imagined as having a a small nucleus where all the positive charge and mass are concentrated with a large space around the nucleus occupied by a cloud of electrons. The slight deflections explained by the repulsion of the positively charged alpha particles by the positive nucleus.

Occasional alpha particle rebounds

A few alpha particles are slightly deflected, repelled by the positive nucleus

Most pass straight through because of the relatively huge space between nuclei

Rutherford’s conclusions

The conclusions from the experiment were:

Most of matter is empty space occupied by electrons of negligible mass and that almost all of the mass of an atom is within a very small positively charged space - the nucleus.

Because

Most of the alpha particles travelled straight through with no deflection, a few were slightly deflected and a very few alpha particles (around one in eight thousand) was deflected by a large angle.

And

Rutherford went on to show that the number of alpha particles deflected at different angles was consistent with the inverse square law for the force of repulsion between the alpha particles and the nucleus.

You can download a PDF of these notes here: Rutherford's gold leaf notes