Model of Atom – Rutherford’s Model

One of the earliest models proposed to describe the structure of an atom was the plum pudding model proposed by J.J Thomson. It states that the atom consists of a homogeneous sphere of positive charges, inside of which are tiny negatively charged particles(electrons). 

To test the validity of this model, Ernest Rutherford experimented in 1911 in which he bombarded a sheet of metal with positively charged alpha particles and examined the deflection pattern. Then he started studying these particles’ pathways after interacting with the gold foil.

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

Rutherford’s Experiment

Rutherford’s model of the atom, also known as a solar or nuclear atom or planetary model of the atom, describes the structure of atoms. New Zealand-born physicist 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.

It is an important experiment that led Rutherford to correctly describe 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 lit. The collimated beam is allowed to fall on a thin gold foil of a thickness of the order of 2.1 × 107m. The scattered alpha particles were observed through a rotatable detector consisting of a Zinc sulphide 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  .

(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) A very few alpha particles were found to be scattered through large angles (greater than 900)

(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 1800.

Result

The large number of alpha particles passing through the atom undeviated 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 a positive charge must be concentrated at the centre of the atom.

A very small number of alpha particles retraced their path indicate that the positive charge in an atom is concentrated in an extremely small space at the centre of an atom.

According to 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 centre of the atom, called a nucleus whose size is of the order of 10-14m.

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

• Atoms as a whole are electrically neutral. The total negative charge on electrons surrounding the nucleus is equal to the total positive charge on the nucleus.

Model of atom – rutherford’s models failed to explain atom stability and linear spectrum of the 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 S coated with zinc sulphide, they produce flashes. These flashes are observed through the telescope T.

Postulates of Rutherford’s Atomic Model 

1. An atom is built of +vely charged particles. The majority of the mass of an atom was concentrated in a very small region. This region of the atom is called the nucleus of an atom. It was later found out that the atom’s tiny and dense nucleus is built of neutrons and protons.

2. An atom’s nucleus is surrounded by -vely charged particles called electrons. The electrons revolve around the nucleus in a fixed circular path at a very high speed. These fixed circular paths are called “orbits.”

3. An atom has no net charge or is electrically neutral because electrons are -vely charged, and the densely concentrated nucleus is +vely  charged. A strong electrostatic force of attraction holds together the nucleus & electrons.

4. The size of the nucleus of an atom is very small as compared to the total size of an atom.

Limitations of Rutherford Atomic Model:

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

The electron stays in orbit in the same way that planets stay in orbit around the sun.

As it turned out, this model was plenty accurate.

The electrons revolving around the nucleus are continuously accelerated towards the centre. According to Lorentz, an accelerated charge 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 can not explain the stability of the atom.

Impact Parameter

Suppose an alpha particle is an incident on a nucleus N and from a large distance. The line drawn through the nucleus N and parallel to the direction of motion of the alpha particle is called the head-on collision line. The perpendicular distance between the line AB and this line is called the impact parameter. It is denoted by b.

Reasons Why It Failed

1. It failed to explain why individual atoms produce discrete visible light spectrum. 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 electron’s kinetic energy would decrease, thereby decreasing the velocity. With a decrease in velocity, the electrons would not stay in orbit and spiral into the nucleus. This implies that atoms are generally stable.

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 one out of every 12000 particles appeared to rebound.

Conclusion

Since many 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 atom’s positive charge occupies very little space.

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