Atoms are considered the smallest units of matter, and many scientists have proposed different models about the structure of these atoms. One such model is the planetary model of an atom, proposed by Ernest Rutherford. He performed an alpha particle scattering experiment to prove his model, and it was a great breakthrough for modern physics.

History Behind the Alpha Particle Scattering Experiment

We all know that, J. J. Thomson was the first person to propose an atomic model called the plum-pudding model. But this model couldn’t explain certain experimental results about the structure of an atom.

Thus, many other scientists performed experiments regarding the structure of an atom, and in this process, Ernest Rutherford, a former student of J. J. Thomson, proposed a new model called the planetary model.

This model was based upon the scattering of alpha particles when a beam of alpha particles hit a thin gold foil (also called the gold foil experiment).

Rutherford’s Planetary Model of an Atom

Ernest Rutherford conducted a gold foil experiment, and based on his findings, he suggested the atomic structure of elements, resulting in the Rutherford atomic model.

Alpha Particle Scattering Experiment

In his alpha particle scattering (gold foil) experiment, Rutherford used alpha particles along with a zinc sulphide covered screen. The alpha particles were fired as a beam of 5.5MeV against a very thin gold foil sheet with 2.1 × 10-7 m thickness.

It was assumed that all of the alpha particles would travel through the gold foil without being deflected. While most alpha particles passed straight through the foil with little deflection, some of the alpha particles were deflected. In fact, the foil bounced some alpha particles nearly straight backwards (almost with a 180⁰ angle).

Alpha Particle Trajectory

When a beam of alpha particles hits something with an initial velocity, we can observe that the particles scatter with different trajectories. As all alpha particles in a beam have the same kinetic energy, the scattering of these particles is completely determined by the impact parameter.

Thus, the trajectory of alpha particles depends solely on the impact parameter of the collision, which is represented by b. In this experiment, it is seen that

• Alpha particles nearer to the nucleus are scattered more as they have less impact factor.
• As for the alpha particles with a higher impact factor, the scattering is less (some particles are not scattered or deviated).
• The particles which underwent head-on collision bounced back as the impact factor was almost zero.

With this experiment, he came to many conclusions and findings.

Observations of Rutherford’s Alpha Particle Scattering Experiment

The observations of Rutherford’s alpha particle scattering experiment are as follows.

• Rutherford, in his experiment, observed that a large amount of the alpha particles passed through the gold sheet with bombardment and were not deflected back. And this can be due to the huge empty space in the atom.
• A small number of alpha particles, when bombarded with the gold sheet, were deflected back with minimal angles, which confirms that the distribution of the positive charges is not uniform.
• The smallest amount of alpha particles were deflected back with an angle of 180⁰, stating that the positively charged particles occupy a very small space in the atom.

Postulates Proposed by Rutherford’s Planetary Model

The following are the postulates proposed by Rutherford after his alpha particle scattering experiment.

• He stated that most of the mass of an atom is at its centre, and this centre is extremely tiny and has a positive charge. He then named this centre as the nucleus (which is why the planetary model is also called the nuclear atom model).
• He also proposed that the negatively charged particles of an atom, which are electrons, will revolve around the nucleus in circular orbits. He also added that these electrons are at a very high speed.
• Finally, he stated that as the centre is very packed and has more density with a positive charge, and electrons with a negative charge revolve around the nucleus. They exhibit a strong, attractive force called the electrostatic force.

Limitations of Rutherford Atomic Model

There are a few limitations of Rutherford’s alpha particles scattering experiment, which are as follows.

• Rutherford didn’t mention anything about the arrangement of electrons in his theory.
• His model has no explanation about the stability of the atoms in accordance with Maxwell’s theory which states that the charged electrons release electromagnetic radiation when excited, resulting in shrinkage of orbits. In contrast, Rutherford stated that the electrons collapse in the nucleus within 8 to 10 seconds.

Conclusion

Though Rutherford’s Planetary model of atoms, which is based on the alpha particle scattering experiment, has some limitations, it is considered to be the most relevant. This was the experiment that was responsible for the discovery of the atomic nucleus. And this brought a great breakthrough in modern physics and also atomic chemistry.