Nowadays, the concept that matter is made of small entities called atoms is widely accepted, which seems very natural to us. However, the description of atoms has changed significantly over history. Ernest Rutherford was the first scientist who attempted to carry out experiments to gain insight into building a well-founded atomic model. He explained the physical phenomenon known as Rutherford scattering.
Rutherford scattering is a type of experiment based on the scattering of particles due to electric interactions with the atoms of a foil.
In the fifth century BC in Ancient Greece, a Greek philosopher named Democritus proposed that matter was made of indivisible entities, which he termed “atoms”. However, his ideas were not accepted, and other models based on universal elements (water, fire) or similar non-scientific features prevailed.
With the advancements of chemistry during the seventeenth century, an English scientist named John Dalton recovered the idea of atoms as basic constituents of matter and developed a set of properties that atoms should have. Although he did not provide further information on the structure of atoms, he paved the way for chemists and physicists to study the different properties of elements and the existence of subatomic particles, such as electrons and protons.
In particular, J.J. Thomson discovered electrons in 1897, and the existence of protons was found shortly after. It was Thomson who proposed that matter is made of atoms. He offered what was known as the “plum pudding model”, which attempted to explain two known properties of atoms. Rutherford came up with an experimental setup (along with Hans Geiger and Ernest Marsden) to investigate this theory further, which is now known as the Rutherford scattering experiment or gold foil experiment.
What is the Rutherford scattering experiment?
The Rutherford scattering experiment was designed to prove Thomson’s model of the atom. Since Thomson proposed a model where matter was made out of atoms that would fill almost all the space available with a positive charge, the goal of Rutherford's experiment was to fire positively-charged particles to study the scattering and test the model.
Rutherford scattering diagram and description
This is what Rutherford used in the experiment:
- A gold foil. For this experiment, Rutherford used a very thin gold foil. (Since gold is very malleable, it is possible to reduce its thickness to 0.00004cm.)
- A beam of alpha particles. The alpha particles were the nuclei of helium (two protons and two neutrons), which, back in the 1910s, were known to have only a positive charge. Given that Rutherford wanted to test the structure of atoms, he considered small positively charged particles he could fire at the gold foil. This is because according to Thomson’s model, atoms were relatively big, positively charged bodies with electrons embedded in them.
- A screen detecting the alpha particles. The screen's purpose was to measure what happened to the alpha particles (their point of impact) after being scattered by the gold foil.
Below is a diagram of the Rutherford scattering experiment:
Rutherford scattering experiment, Wikimedia Commons
What were the results of the Rutherford scattering experiment?
The mechanism in the experiment is relatively simple. By firing alpha particles against the gold foil and detecting where they end up, we can extract important conclusions about the atomic structure of the gold’s atom. When conducting this experiment, it is important to reduce the thickness of the foil as much as possible because this prevents multiple scattering events that could compromise the conclusions of the experiment. Ideally, each alpha particle is supposed to interact with only one gold atom.
Conclusions from the experiment
When Rutherford did the experiment, he expected to detect most of the alpha particles on the side closer to the alpha emitter. This was because Thomson's model was based on large chunks of positive charge that would repel the alpha particles (backwards). However, the experiment yielded the opposite results. Most alpha particles travelled through the gold foil and were not scattered, with a few particles scattering slightly. Only a small number of particles were scattered strongly, and Rutherford observed that the bigger the scattering angle was, the lower the number of alpha particles.
This is what the experiment concluded:
- Matter is almost empty. Since most of the alpha particles did not deviate from their original trajectory, Rutherford deduced that the spaces between atoms are big compared to the size of atoms themselves.
- Since only a few particles were strongly scattered (repelled) by the gold atoms, Rutherford concluded that atoms must have a part where the positive charge is concentrated. He called this part the nucleus.
- Since the fraction of alpha particles that were strongly scattered was very small, Rutherford also deduced that the size of the nucleus has to be very small. He based this conclusion on the electric properties of charged bodies and the forces they exert on others.
- After several scattering experiments, Rutherford created a theoretical model in which most of the atom’s mass was concentrated in the nucleus. Since his observations fitted the predictions of this model, he was able to deduce this feature of atoms.
In addition, Rutherford reached the following conclusion by using previous knowledge and the results of his experiments:
- Electrons orbit the nucleus. Since atoms are neutrally charged, electrons must balance the positive charge of the nucleus. Also, the electrons cannot be too close to the nucleus, otherwise they would not scatter alpha particles. Since they cannot be close to it, electrons are forced to orbit the nucleus (and are thus moving). If they didn’t orbit the nucleus, they would fall towards the nucleus due to electrostatic attraction.
Problems with the experiment
These characteristics were very different from the ones of the Thomson atomic model, and Rutherford’s model was the first atomic model fully based on experimental evidence. However, problems with both the experimental method and the model itself needed to be solved. Here are some of those problems:
- The thickness of the foil could affect the scattering pattern significantly, therefore offering biased conclusions.
- After Einstein developed his Theory of Relativity, it was discovered that charges in movement continuously radiate energy. If electrons orbit the nucleus, how can they not lose all their energy due to radiation and fall towards the nucleus?
The first issue was irrelevant: the scales of the thickness of the foils used were enough to obtain reliable scattering results. However, the second issue raised a lot of concerns, which were later solved with the introduction of quantum physics, the Bohr atomic model, and the quantum atomic model.
There is another issue with the Rutherford scattering experiment that was not known back then. Since the existence of protons was intuited but not known, the models provided no further structure of the nucleus apart from charge and mass considerations. Later, it was discovered that subatomic particles called protons carry a positive electric charge.
Due to electric repulsion among protons, a type of particle was predicted to shield their interactions in the nucleus. This particle is the neutron. The shielding of the interaction had to be because of another force between neutrons and protons (what we now know as strong force).
This force plays a role in scattering alpha particles because alpha particles are themselves made of neutrons and protons. However, until the model of this force was fully established, it was not known that most of the effects observed in Rutherford scattering are actually due to the electric force and not the strong force.
Rutherford’s atomic model
The force causing the scattering is the electric force of repulsion between gold nuclei and alpha particles. However, we must still consider the usually irrelevant (but present) effect of the strong force.
Rutherford Scattering - Key takeaways
- Rutherford conducted a series of experiments of scattering to obtain experimental evidence on the characteristics of an atomic model.
- The experiment was based on the scattering of alpha particles due to the presence of a gold foil. The scattering was produced by the electrostatic interaction between alpha particles and gold nuclei.
- The results of the experiments contradicted the atomic model developed by Thomson and yielded the existence of a small nucleus.
- The model developed by Rutherford had some inconsistencies that were solved afterwards thanks to quantum physics. Nevertheless, it was an excellent atomic model that was fully based on experimental evidence for the first time in history.
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