What is Rutherford’s Gold Foil Experiment

Rutherford’s gold foil experiment (Rutherford’s alpha particle scattering experiment) refers to an experiment carried out by Ernest Rutherford, Hans Geiger, and Ernest Marsden at the University of Manchester in the early 1900s. In the experiment, Rutherford and his two students studied how alpha particles fired at a thin piece of gold foil were deflected. According to the popular atomic models of the time, all of the alpha particles should have traveled straight through the gold foil. However, to their surprise, Rutherford and his students found that around 1 in every 8000 alpha particles were deflected back towards the source (i.e. at angles larger than 90o). In order to explain this effect, they had to come up with a new model (now known as the “Rutherford Model“) for the atom.

What is Rutherford's Gold Foil Experiment - Ernest Rutherford

Ernest Rutherford

For the experiment, a radioactive source which emits alpha particles was kept in front of a thin gold foil. The source and the gold foil was surrounded by a screen with a zinc sulphide coating, and the air was pumped out to ensure that the equipment were all within a vacuum. (If they had not, the alpha particles would have used up their energy to ionise air molecules and may have never reached the gold foil).

The alpha particles emitted by the source were expected to pass straight through the gold foil. Whenever they hit the zinc sulphide coated screen, they were to produce a small glowing spot on the screen.

The popular model for the atom at the time was known as the “Plum Pudding Model“. This was a model developed by J.J. Thomson, who had discovered electrons a few years earlier. According to his model, atoms were spherical objects, with the positive charge evenly spread throughout like a dough, and little bits of negative charge (electrons) sticking on it like plums. If this “Plum Pudding Model” had been correct, all of the alpha particles should have passed straight through the gold atoms in the gold foil, showing very little deflection. However, what Rutherford and his students observed was quite different.

Most of the alpha particles did go straight through the gold foil. However, a few of the alpha particles seemed to be deflected at large angles. Rarely, some alpha particles even seemed to have been deflected by angles larger than 900. To explain this result, Rutherford proposed that the mass of an atom must be concentrated in a very small area at the centre, which he called the “nucleus”. From the deflections, it was also clear that the nucleus was charged:

What is Rutherford's Gold Foil Experiment - Experiment Setup and Observations

Rutherford’s Gold Foil Experiment – Geiger-Marsden experiment expectation and result

 

Rutherford’s Gold Foil Experiment – Main Observations and Conclusions

Observation Interpretation
Most of the alpha particles passed straight through the gold foil These alpha particles must be traveling without getting close to the (charged) centre of the atom. Therefore, most of the atom must be empty.
Few of the alpha particles were deflected at large angles These must be coming close to the centre of the atom, where they get deflected from the charge at the centre. So, the nucleus must be charged.
Rarely, alpha particles were deflected back towards the detector These must have collided with the nucleus head-on. So, the nucleus must contain most of the atom’s mass.

Rutherford did not necessarily determine that the nucleus was positively charged during these early experiments (the deflections could have been produced by attractive negative charges rather than repulsive positive charges at the centre). Rutherford eventually did discover that the nucleus of an atom was positively charged, but this was done in a different experiment.

Eventually, Niels Bohr and Erwin Schrödinger came up with better models for atoms, but Rutherford’s gold foil experiment remains one of the most groundbreaking experiments in the history of physics.

 

Image Courtesy:
1. “Ernest Rutherford 1892″ by Unknown, published in 1939 in Rutherford: being the life and letters of the Rt. Hon. Lord Rutherford, O. M [CC BY 4.0], via Wikimedia Commons
2. “Geiger-Marsden experiment expectation and result” by Kurzon (Own work) [CC BY-SA 3.0], via Wikimedia Commons

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