Rutherford Scattering Experiment
- Updated by Scienly
- On September 10, 2024
In this chapter, we will understand Rutherford’s scattering experiment and its observations and conclusions. In order to understand the arrangement of charged particles like electrons and protons in an atom, the British scientist, Ernest Rutherford and his co-workers in 1911 carried out a series of experiments using alpha particles known as Rutherford’s alpha scattering experiments. This experiment is based on the alpha particles (helium nuclei) experiment.
The Rutherford’s alpha particle scattering experiment is one of the most important experiments in the history of atomic physics. It laid the foundation for the modern understanding of the atom, challenging earlier models and introducing the concept of an atomic nucleus.
Before Rutherford’s experiment, J. J. Thomson had given the first model of an atom in 1904, also known as “plum pudding model of atom”. In this model, he proposed that an atom may be considered as a sphere of positively charge particles in which the negatively charged particles called electrons are embedded to make the atom as a whole neutral.
This model was discarded in 1911 on both theoretical and experimental due to the Rutherford’s atomic model. Now let’s understand Rutherford scattering experiment step by step.
Rutherford’s Alpha Particle Scattering Experiment
In this experiment, Ernest Rutherford bombarded a beam of alpha particles with a very thin gold foil. The thickness of gold foil is approximately 0.00006 cm. The alpha particles were emitted from the radioactive substances such as radium or polonium. An alpha particle is a doubly ionized helium atoms or ions with two units of positive charge, mass number is equal to 4 and missing two electrons. It is represented as +2 He 4 or +2 α 4 .
The pictorial representation of Rutherford’s scattering experiment is shown in the below figure.
From the above figure, it is clear that he produced a narrow beam of alpha particles from a radioactive substance like radium placed in a lead block and then passed it through a thin sheet of gold foil. A movable circular screen coated with Zinc Sulphide (ZnS) screen is placed around the gold foil in order to detect alpha particles after scattering.
When these alpha particles hit the screen, it produced a tiny flash of light (scintillation) on the screen, which could be observed through movable a microscope.
Observations from Alpha Particle Scattering Experiment
The following observations were made from the alpha particle scattering experiment:
- Most of the alpha particles (nearly 99%) passed through the gold foil with little to no deflection. In other words, most of the alpha particle moved in the straight path with no deflection. This means that most of the space inside the atom is empty, as shown in the above figure.
- Some of the alpha particles deflected with small angle, meaning that the positive charge of an atom occupies a tiny space.
- A very few alpha particles (approx. 1 in 20,000) deflected back through the deflection of 180 degrees. This means that the whole positive charge and mass of the atom is concentrated in a very small volume inside the atom.
Thomson’s atomic model could not explain these observations. According to Thomson’s model of atom, positive charge and mass inside an atom are uniformly distributed throughout its volume.
Conclusion from Rutherford Scattering Experiment
After performing a series of scattering experiments, Rutherford concluded that:
- Most of the space inside an atom is empty because most of the alpha particles passed through the gold foil undeflected.
- There must be the presence of a heavy positively charged mass at the center of an atom because some of the alpha particles are deflected by a certain angle.
- A heavy positively charged mass at the center of the atom is very small, which he named as nucleus.
On the basis of these observations, Sir Ernest Rutherford proposed the nuclear model of atom, which you will study in the next chapter.
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