Rutherford's Model of Atom— Discovery of Nucleus

Introduction to Rutherford's Model of Atom

Rutherford, in 1911, perform some scattering experiments in which he bombarded thin foils (nearly 100 nm thick) of metals like gold, silver, platinum or copper with a beam of fast moving Alpha particles. The source of Alpha particles was radium, a radioactive substance, placed in a block of lead. Slits were used to get a fine beam.

The presence of Alpha particle at any point around the thin foil of gold after striking, it was detected with the help of circular zinc sulphide screen. The point at which Alpha particle strikes this screen, a flash of light is given out.

Observations made from Experiment

From these experiments, he made the following observations:-

• Most of the Alpha particles (99.9%) passed through the foil without undergoing any deflection.

• Few Alpha particles underwent deflection through small angles.

• Very few (only one in 20,000) were deflected back that is through an angle greater than 90 degree.

Conclusions made from Experiment

From these observations, Rutherford drew the following conclusions:-

• Since most of the Alpha particles passed through the foil without undergoing any deflection, there must be sufficient empty space within the atom.

• Since few Alpha particles were deflected through small angle and Alpha particles were positively charged particles, these could be deflected only by some positive body present within the atom. The Alpha particles deflected were those which passed very close to the positive body.

• Since some Alpha particles were deflected back and Alpha particles are heavy particles, these could be deflected back only when they strike some heavier body inside the atom.

• Since the number of Alpha particles deflected back is very very small, this shows that the heavy body present in the atom must be occupying is a very very small volume.

The small heavy positively charged body present within the atom was called nucleus.

Discovery of Nucleus 

In the light of above results, the scattering of Alpha particles according to the Rutherford's model of atom may be depicted as shown in figure.

 Thus according to Rutherford's model of atom, the atom consist of two parts:-

1. Nucleus

It is very small in size, carriers positive charge and in which the entire mass of the atom is concentrated.

Since electrons have negligible mass, the mass of the atom is mainly due to the protons and neutrons. Hence, protons and neutrons must be present in the nucleus. The presence of positively charged protons in the nucleus also accounts for the positive charge on the nucleus.

2. Extra-Nuclear Part

The space around the nucleus in which the electrons were distributed.
To explain that the electrons do not fall into the nucleus as a result of attraction, Rutherford suggested that the electron was not stationary, but were revolving around the nucleus in certain circular orbits. As a result, centrifugal force come into play which balance is the force of attraction.

Thus, this model of atom is similar to our solar system where the nucleus is like sun and the electrons are like planets. That is why these electrons are called planetary electrons.

Drawbacks of Rutherford's Model  of Atom

1. Inability to Explain the Stability of Atom

According to Rutherford model, an atom consists of a small, heavy positively charged nucleus in the centre and the electrons were revolving around it. This model was compared with the solar system in which the planets revolving around the sun and continue to move in their fixed circular paths because the force of attraction was balance by the centrifugal force.

However, there are some important differences between the two. Firstly, whereas the electrons and the nuclei are microscopic particles, the sun and the planets were big bodies and hence Newton laws of motion do not applied to the motion of microscopic particles.

Secondly, whereas the electrons and the nucleus were charged particles, the sun and the planets did not carry any charge. According to Maxwell's electromagnetic theory, whenever a charged particle like electron is revolving in a field of force like that of nucleus, it loses energy continuously in the form of electromagnetic radiation. This is because when particle is revolving, it undergoes acceleration due to change in direction even if the speed remains constant.

Thus, the orbit of revolving electron will keep on becoming smaller and smaller, following is spiral path as shown in figure and ultimately the electron should fall into the nucleus. In other words, the atom should collapse. However, this actually doesn't happen and the atom is quite stable.

Thus, Rutherford model could not explain the stability of atom.

2. Inability to Explain the Line Spectra of Elements

It could not explain why elements produce line spectra.

3. Inability to Describe Distribution of Electrons and Energies of Electrons

It was unable to explain how the electrons are distributed around the nucleus and what were their energies.


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