The alpha-particle scattering experiment is an important experiment that led Rutherford to give an accurate description of the distribution of positive and negative charges within the atom. Alpha particle source is placed in the lead cavity. The alpha particles emitted by the source are collimated into a narrow beam with the help of lead and slit. The collimated beam is allowed to fall on a thin gold foil of thickness of the order of 2.1 × 107m. The scattered alpha particles were observed through a rotatable detector consisting of a zinc sulfide screen and a microscope. The alpha particles on striking the screen produce scintillations, which could be observed and counted at different angles from the direction of the incident beam.

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It is an important experiment that led Rutherford to give a correct description of the distribution of positive and negative charges within the atom.

Experimental observations

A graph is plotted between the scattering angle and the number of alpha particles N() scattered < . 

From the graph, Rutherford and his associates made the following observations:

(i) Most of the alpha particles pass straight through the gold foil or were found to be deflected through small angles, and their angular distribution is fixed.

(ii) Very few alpha particles were found to be scattered through large angles (greater than 90°).

(iii) A very small number of alpha particles, about 1 in 8,000, practically retraced their paths.

(iv) An alpha particle rarely renounces, i.e., it is scattered through an angle of 180°.

Rutherford atomic model:

On the basis of the results of his famous alpha-particle scattering experiment, Rutherford proposed the following model for the structure of the atom:

• Atom may be regarded as a sphere of radius 10-10m in which the whole of the positive charge and almost the whole mass of the atom is concentrated in a very small region at the center of the atom, called a nucleus whose size is of the order of 10-14m.

• A suitable number of electrons revolve around the nucleus in circular orbits of all possible radii because of the necessary centripetal force of attraction between electrons and nucleus.

• As atoms, on the whole, are electrically neutral, the total negative charge on electrons surrounding the nucleus is equal to the total positive charge on the nucleus.

Rutherford’s model of atoms failed in explaining atomic stability and the linear spectrum of an atom.

The radium source emits alpha particles in all directions. Most alpha particles are absorbed by the lead block. Only a thin pencil of alpha rays comes out of the tunnel and falls on the gold foil. The gold atoms scatter the alpha particles. When scattered alpha particles fall on a screen coated with zinc sulfide, they produce flashes. These flashes are observed through the telescope.

The assumptions made by the Rutherford are as follows:

• He considered that an atom is made by positively charged particles and at the center most of the mass of an atom is concentrated. This concentrated region is called the nucleus of the atom. Later it was found out that the nucleus has protons and neutrons and the nucleus is very dense.

• Negatively charged particles called electrons revolve around the nucleus in a fixed circular path at a very high speed. The fixed circular paths of electrons are called “orbits.”

• An atom is electrically neutral  because electrons are negatively charged, and the protons are positively charged. The nucleus and electrons are held together by a strong electrostatic force of attractions.

Limitations of the Rutherford atomic model:

Rutherford’s model suffered from the following limitations:

1. A revolving electron, which was proposed by Bohr, is continuously accelerated due to a change in direction. According to electromagnetic theory, an accelerated electron must radiate energy continuously. This should decrease the energy of the electron, and thus, the electron should follow a spiral path of decreasing radius till it falls into the nucleus. Hence, such orbits can not be stable.

2. The spiraling electron must emit radiation of decreasing wavelength continuously, which is contrary to experimental observation because the spectral lines observed are discrete.

The reasons for the above limitations are:

1. It failed to explain why individual atoms produce discrete spectra of visible light. According to Rutherford’s model, when electrons accelerate in orbit, they should produce electromagnetic radiation over a wide range of frequencies. Therefore, they should produce a continuous light spectrum. However, experiments show that individual atoms produce discrete line spectra.

2. According to Rutherford’s model, the electron accelerates around in a circular orbit. According to Maxwell’s theory of electromagnetism, this electron will emit radiation in the form of light. Therefore, the kinetic energy of the electron would decrease, thereby decreasing the velocity. With a decrease in velocity, the electrons would not stay in orbit and would spiral into the nucleus. 

Observations: 

Most of the fast-moving alpha particles pass straight through the gold foil.

Some of the alpha particles were deflected by the foil by small angles.

Surprisingly 1 out of every 12,000 particles appeared to rebound.

Conclusion:

1. A large number of alpha particles passing without deviation through the atom indicate that most of the portion of the atom is hollow inside.

2. The scattering of alpha particles through small angles indicates that the position of the atom responsible for scattering must also have a positive charge. Further, the angular distribution of the scattered alpha particles was fixed, hence, it shows that positive charge must be concentrated at the center of the atom.

3. A very small number of alpha particles retraced their path, indicating that the positive charge in an atom is concentrated in an extremely small space at the center of an atom.