Rutherford’s Gold Foil Experiment

Ernest Rutherford is credited with proving that atoms have a small, dense and positively charged nucleus surrounded by negatively charged electrons.

To be exact, it was Ernest Marsden who carried out the very first version of the famous gold foil experiment while working under the direction of Rutherford and Hans Geiger. Rutherford was not even the first person to suggest that atoms contain a lot of empty space (Hantaro Nagaoka, working separately in Japan, proposed a similar idea several years earlier) but despite all of this, Rutherford’s ideas, formulated in 1909 and published in 1911, were little short of revolutionary after hundreds of years when the accepted model of the atom had been based on a small, solid sphere.

To fully appreciate the importance of Rutherford’s work it is necessary to elaborate on the “small, solid sphere” model mentioned above.

It is often said that the idea of an atom, as the smallest part of any pure substance, originated in Ancient Greece but for hundreds of years this idea was just the basis for philosophical discussions: it really didn’t matter whether there was a “smallest part” or not! Only when chemists found that different elements combine in fixed ratios to make new substances did the idea of atoms start to have any significance. That was at the very end of the 18th Century and was due to the efforts of Joseph Proust in Spain, Joseph Priestley in England and Antoine Lavoisier in France. In 1803, John Dalton built on this work to develop his ideas about atomic theory and the concept of a “small, solid sphere” was born.

It was assumed that the sphere was uniform throughout but this was proved not to be the case when J J Thomson discovered that “cathode rays” were in fact small, moveable particles called electrons that had come from atoms. A year later he devised a version of the atom that is often known as the “currant bun” or “plum pudding” model as it has discrete lumps (electrons) spread through a uniform medium. Given that electrons were known to be negatively charged, and atoms are electrically neutral overall, it was clear that the rest of the atom had to contain an amount of positive charge that would exactly balance the electrons’ negative charge. Since there were no positive particles known at that time (although some physicists, such as Philipp von Lenard, thought they might exist) the overall opinion was that the positive charge took the form of a general cloud surrounding the negative electrons.

It was this “cloud” that Rutherford’s team set out to investigate by firing a beam of alpha particles (helium nuclei) towards a thin gold foil. A diagram showing this famous experiment is shown below. According to Thomson’s model of the atom, most of the alpha particles should go straight through but, as the illustration shows, that was not the only result!

The important points of the gold foil experiment are as follows;

• Most of the alpha particles went straight through as expected for the plum pudding model: in Rutherford’s version, this was evidence for the majority of the atom being empty space.
• Some of the alpha particles were deflected through a range of different angles: in Rutherford’s model, this was due to the nucleus having a positive charge that repelled the positively-charged alpha particles by different amounts as they went through the gold foil at different distances from the nucleus.
• A very small number of alpha particles bounced back in a way that could not be explained by the plum pudding model: in Rutherford’s model, this proved that the positively-charged nucleus must occupy only a tiny fraction of the space of an atom as the chances of achieving a “direct hit” were clearly very low.

Taken together, these results led to the nuclear model of the atom. In fact it was Rutherford who first used the word “nucleus” to label the tiny, central part of the atom that is very dense and positively-charged.

There is a great five-minute video that explains all of this, courtesy of fizzics.org. I strongly recommend that you take a look at the video, which you can view on YouTube, by clicking here.

There is also a simple animation you can watch. The speed of the alpha particles can be adjusted but apart from that it’s just something for you to view to ensure that you’re familiar with the basic principles of the experiment’s results. To access the applet, which comes courtesy of Michael Fowler’s Galileo and Einstein website at the University of Virginia, click here.

As a footnote, it is interesting to note that Rutherford won the Nobel prize for chemistry (not physics) in 1908 after discovering that uranium released two different types of radiation, which he named alpha and beta. He had previously shown that gamma waves are similar to x-rays but with a shorter wavelength. Rutherford also suggested and went on to prove, when working with Hans Geiger, that alpha particles are simply the nuclei of helium atoms.

It should also be noted that having the positive charge of an atom confined to a very small volume raises the question, how is this possible: surely the positive charges would repel each other and cause the nucleus to disintegrate? The fact that the nucleus also contains neutrons (which Rutherford didn’t know) offers a possible mechanism for holding everything together but it wasn’t until Hideki Yukawa developed his meson theory that things were properly explained. Even that isn’t the full story but you can read a short account of Hideki Yukawa’s theory elsewhere on physbang (https://physbang.com/2024/01/19/hideki-yukawa-and-meson-theory/).