Rutherford Alpha Scattering Experiment

In this post we will discuss about Rutherford and Thomson Model of an atom.

After the discovery of electrons and protons, Sir J.J. Thomson proposed the atomic model. This is described below.

What is J J Thomson Model of the atom?

In 1898, Sir J J Thomson proposed a model of the atom. This model is known as J J Thomson’s plum pudding model of the atom.

According to this model,

• An atom is considered to be a sphere of uniform positive charge, and electrons are embedded into it like raisins in a plum pudding.

• In an atom, total positive charge is equal to total negative charge due to the electrons.
• Mass of the atom is considered to be uniformly distributed.

What are the limitations of Thomson’s atomic model?

The limitations of Thomson’s model of the atom are described below:

• Thomson’s model of an atom considered an atom to be a sphere of uniform positive charge. Later researchers particularly Rutherford’s alpha-particle scattering experiment show that in an atom positive charge is concentrated in a very small volume at its centre. This small volume at the centre of the atom was called the core.
• According to Thomson’s model the mass of an atom is considered to be uniformly distributed. Rutherford’s experiment showed that the entire mass of an atom is concentrated at its centre.

What is Rutherford’s alpha scattering experiment?

In 1911, Rutherford performed his famous alpha-particle scattering experiment.

In his experiment, Rutherford bombarded at thin sheet (0.000006 cm thick) of gold foil with alpha particles in an evacuated chamber. A simplified picture of an alpha scattering experiment by a thin gold foil is shown below:

The following observations are made of the results obtained:

• Most of the alpha particles passed through the foil straight without suffering any change in their direction.
• A small fraction of alpha particle was deflected through small angles, and a few through larger angles.
• One particle in every 10,000 returned back. This observation surprised everyone in the laboratory.

This discovery was so fascinating that Rutherford wrote the following comments regarding the scattering experiment.

What conclusions were derived from the Rutherford alpha scattering experiment?

The following conclusions were drawn from the Rutherford’s scattering experiment.

• Most of the alpha particles passed straight through the foil without suffering any deflection. This showed that most of the space inside the atom is empty or hollow.
• Some of the alpha particles suffered deflection by 90 or even larger angles. For this to happen, alpha particles (positively charged) must approach a heavily positively charged core inside the atom (like charges repel each other). This heavy positively charged core inside the atom was named as nucleus.
• Since the number of alpha particles which bounced back was very small, hence the volume occupied by the nucleus is very small as compared to total volume of the atom.
• The alpha particles have appreciable mass and are deflected by the nucleus, It means almost the entire mass of the atom lies at its centre, i.e. the nucleus.

Rutherford’s Nuclear Model of an atom

On the basis of the famous alpha particle scattering experiment, Rutherford proposed his nuclear model of the atom in 1911. The main points of the Rutherford’s atomic model are described as follows:

• An atom consists of a positively charged nucleus, which is surrounded by the electrons moving around it.
• Electrons and the nucleus are held together by coulombic force of attraction.
• The size of the nucleus is very small as compared to the size of the atom.

Experimentally it was found that,

Radius of the nucleus of an atom = 10^-15 m

Radius of an atom = 10^-10 m

Thus, the size of the nucleus is about 10,000^th part of the size of an atom.

• Almost the entire mass of an atom is concentrated in its nucleus.
• Atom, as a whole, is electrically neutral. So, number of proton inside the nucleus of an atom and the number of electrons surrounding the nucleus are equal.

What are the drawbacks in the Rutherford nuclear model of an atom?

The Rutherford’s atomic model explains the structure of an atom in a very simple way. But it suffers from the following drawbacks:

• An electron revolving around the nucleus gets accelerated towards the nucleus. An accelerating charged particle must emit radiation, and lose energy. Thus, the electrons in an atom must continuously emit radiation and lose energy. Because of this loss of energy, the electron would slow down and will not be able to withstand the attraction of the nucleus. As a result, the electron should follow a spiral path, and ultimately fall into the nucleus. If it happens, then the atom should collapse in about 10^-8 second. But, this does not happen – atom is stable. This indicates that there is something wrong in

• Rutherford’s nuclear model of atom. Rutherford’s model of atom does not say anything about the arrangement of electron in an atom.

Read More: Cathode Rays and Discovery of Electron

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