One of the earliest models proposed to describe the structure of an atom was the plum pudding model proposed by J.J. Thomson. It described the atom consisting of a homogeneous sphere of positive charge, inside of which are tiny negatively charged particles called electrons. 

In order to test the validity of this model, Ernest Rutherford conducted an experiment in 1911, in which he bombarded a thin sheet of gold with positively charged alpha particles and examined the deflection pattern. 

Rutherford, in his experiment, projected high-energy streams of alpha particles from a radioactive source at a thin sheet of gold (usually 100 nm thickness). In order to study the deviation caused to the alpha particles, he placed a fluorescent zinc sulfide screen around this thin gold foil. Rutherford made certain observations that contradicted Thomson’s atomic model and gave a new model called Rutherford’s atomic model.

Body

Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. He designed an experiment for this. In this experiment, fast-moving alpha particles were made to fall on a thin gold foil. Rutherford’s model of the atom, also known as solar or nuclear atom or planetary model of the atom, describes the structure of atoms proposed by the New Zealand-born physicist Ernest Rutherford.

It is an important experiment that led Rutherford to give a correct 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 × 10-7m. 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.

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 8000, practically retraced their paths.

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

Result

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

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.

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.

On the basis of Rutherford’s famous alpha particle scattering experiment called Rutherford’s alpha scattering experiment,

• 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 in 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 atom failed to explain 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.

Postulates of Rutherford’s atomic model 

• An atom’s majority mass is concentrated at the center in a very small region. This region is defined as the nucleus of the atom. Later it is found that in the nucleus two types of particles are present i.e. proton and neutron. 

• In an atom’s nucleus, total positive charge is stored. Negatively charged electrons revolve around the nucleus in circular orbits. 

• An atom is electrically neutral because the nucleus is positively charged and electrons are negatively charged. There is electrostatic force between electrons and nucleus. 

• The nucleus is very small compared to the size of an atom. 

Limitations of Rutherford atomic model:

Following his experiment, Rutherford proposed the planetary model of atoms. In this mode, the atom consists of a tiny but very concentrated region of a positively charged nucleus surrounded by orbiting electrons. He argued correctly that the atom is mostly empty space.

The electron stays in orbit in the same way that planets orbit around the sun. As it turned out, this model was plenty accurate.

The electrons revolving around the nucleus are continuously accelerated towards the center. According to Lorentz, an accelerated charged particle should continuously radiate energy. Therefore, in an atom also, a revolving electron should continuously emit energy. Hence, the radius of its path goes on decreasing, and ultimately, it should fall into the nucleus.

Therefore, the Rutherford atomic model could not explain the stability of the atom.

Reasons of failure of Rutherford atomic model

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. By Maxwell’s theory of electromagnetism, this electron will emit energy 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. 

Conclusion

Since a large number of alpha particles suffer practically no deviation, it means that a major portion of an atom is empty space.

Very few particles deflect from their path, indicating that the positive charge of the atom occupies very little space.

The backward scattering of alpha particles shows that most of the atom is concentrated in a small portion of its volume.