Rutherford’s Model of an Atom

One of the earliest models describing 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 enclosing tiny negatively charged particles (electrons).

In 1911, New Zealand-born physicist Ernest Rutherford tested the validity of the Thomson model. Rutherford made specific observations contradicting Thomson’s atomic model, putting forth a new model called Rutherford’s atomic model. The other names for this model include the solar model, nuclear atom model and planetary model.

Rutherford’s Experiment

Rutherford was interested in knowing the arrangement of the electrons within an atom. Therefore, he designed an experiment wherein he bombarded high-energy streams of alpha particles from a radioactive source at a thin sheet of gold.

He placed the source in a lead cavity, wherein the emitted alpha particles transform into a narrow beam with the help of lead. He then directed this beam onto a thin gold foil of a thickness of the order of 2.1 × 10-7 m.

To study the deviation caused to the alpha particles, he placed a rotatable detector around the thin gold foil. This detector contained a microscope and a fluorescent zinc sulphide screen.

When the alpha particles hit the screen, they produce scintillations. Rutherford observed and counted these flashes at different angles from the direction of the incident beam.

This crucial experiment enabled Rutherford to correctly describe the distribution of positive and negative charges within the atom.

Observations

Rutherford plotted a graph representing the scattering angle and the number of scattered alpha particles N().

1. Most alpha particles penetrate through the gold foil with fixed angular distribution.
2. Some alpha particles scatter through large angles (greater than 90o). These particles were very few.
3. The foil deflects some alpha particles by small angles.
4. About 1 in 8000 practically retrace their paths.
5. An alpha particle rarely renounces, i.e., scatters through an angle of 180o.
6. Surprisingly, one out of every 12000 particles appears to rebound.

Result

The results of Rutherford’s experiment are as follows:

• The large number of alpha particles passing undeviating through the atom indicates that the atom is primarily hollow inside.
• The alpha particles scatter through small angles with fixed angular distribution. Further, only a few alpha particles retraced their path. These behaviours indicate a concentration of positive charge in a small space in the atom’s centre.
• Rutherford’s alpha scattering experiment infers that:
  • We may regard the atom as a sphere of radius 〜10-10 m. In this sphere, a tiny region in the centre comprises the entire positive charge and almost the whole mass of the atom. This centre of the atom is the nucleus, whose size is of the order of 10-14 m.
  • A suitable number of electrons revolve around the nucleus in circular orbits of all possible radii. The necessary centripetal force for this revolution is the attraction between electrons and the 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.

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. His model drew similarities with the solar system model. He argued correctly that the atom is mostly space.

Postulates of Rutherford’s Atomic Model

1. An atom consists of positively charged particles. The atom has a nucleus, a tiny region at its centre that comprises most of its mass. Later research established that neutrons and protons make up this nucleus.
2. Negatively charged particles called electrons surround the nucleus, bound by strong electrostatic forces of attraction. They revolve around it in a fixed circular path, called “orbits”, at a very high speed.
3. An atom carries no net charge. The negative charge of the electrons cancels the positive charge of the nucleus, making the atom electrically neutral.
4. The size of the nucleus of an atom is very small as compared to the total size of an 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 for Failure of Rutherford’s Atomic Model

Model of an atom: Rutherford’s model failed to explain atom stability and the linear spectrum of an atom.

• Linear Spectrum of an Atom

The model failed to explain why individual atoms produce a discrete visible light spectrum. When electrons accelerate in orbit, they should produce electromagnetic radiation over a wide range of frequencies. Therefore, they should create a continuous light spectrum. However, experiments show that individual atoms have discrete line spectra.

• Atomic Stability

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. This constant energy emission would decrease the electron’s kinetic energy, reducing the velocity. Ultimately, the electrons would not stay in orbit and spiral into the nucleus. However, the reality implies that atoms are generally stable. Hence, Rutherford’s atomic model can not explain the stability of the atom.

Conclusion

Rutherford’s model of an atom is among the classic models proposed to describe the structure and composition of an atom. Ernest Rutherford proposed this model based on the inferences from his alpha scattering experiment.

The main results from the Rutherford model were that atoms had a positively charged nucleus surrounded by negatively charged electrons. The model served as a foundation for later research on atomic structures.