Plum Pudding Model

Ernest Rutherford’s gold foil experiment proved that most of the interior of an atom is comprised of empty space with most of the mass concentrated in a small dense nucleus. The experiment itself also referred to as the Geiger-Marsden experiment or the Rutherford experiment was conducted by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford.

The experiment, conducted in 1909, was expected to provide information about the distribution of charge within the atom. At this time the Thomson plum pudding model was the accepted model for the interior of an atom. Thomson, who had discovered the electron while working with cathode rays, had explained the neutral charge of an atom by theorizing that electrons were evenly scattered within a positively charged medium.

Geiger and Marsden bombarded a very thin sheet of gold foil with alpha particles provided by the radioactive decay of radium. A zinc sulfide sheet that would give off light when hit with alpha particles surrounded the gold foil and was used to detect the angle of deflection of the particles. Due to Thomson’s theory that the positively and negatively charged parts of the atom where evenly distributed it was expected that most of the particles would go through while those that didn’t would be deflected by a few degrees. By carefully measuring the pattern of the deflections it was expected that information about the distribution of charge within the atom could be found.

The expectations for the experiment were not realized but instead while most of the particles passed straight through the foil as expected a small percentage were deflected at much larger angles then expected and some were deflected directly back. Due to the fact that alpha particles had about 8000 times the mass of an electron and yet struck the foil at very high velocities, something other then an electron was causing the backscatter and the large degree deflections.

According to Rutherford,

“….On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea that the mass of an atom was concentrated in a minute nucleus. … carrying a charge.

Thus the Thomson (plum pudding model) was, because of the gold foil experiment, discarded and the Rutherford (planetary) model was adopted. Rutherford wrote about the experiment and his conclusion in a paper published in 1911 and, although his model wasn’t entirely correct with regard to the electron charge placement and motion, his conclusions from the gold foil experiment concerning the existence of the nucleus still holds true.