Rutherford Scattering Experiment and Atomic Model for JEE

VSAT 2022

Introduction to Rutherford Alpha Particle Scattering Experiment

In this article, we will look into the famous Rutherford scattering experiment. The experiment was performed to explain the atomic structure of elements. There are many atomic models introduced earlier before it, such as the “Plum pudding model” by British Physicist J.J. Thomson. In this model, he compared the atom to a plum pudding in which the negative charge is the dry fruits and the positive charge is spread all over. The atom is neutral with an equal number of positive and negative charges. But this model failed to explain experimental results about the atomic structure completely.

The Rutherford alpha particle scattering experiment was discovered by Ernest Rutherford, a New Zealand Physicist. He was interested in studying the atomic structure and designed the scattering experiment. Rutherford is famous for his pioneering studies of radioactivity atoms other than the alpha particle scattering experiment, and he is known as the father of Nuclear Physics.


Experimental Setup of Rutherford Alpha Scattering Experiment

Rutherford's alpha-particle scattering experiment was a successful attempt to present an atomic model. In the experiment, Rutherford used an alpha particle emitter, gold foil, and a movable fluorescent screen proper description of the same is given below:

Gold foil: A thin sheet of gold foil with a thickness of around 100 nm.

Alpha particle emitter: A high-energy beam of α-particles (Helium ions) emitted from the radioactive source.

Movable Fluorescent screen: A zinc sulphide screen is placed around the gold foil. 

The Rutherford scattering experiment diagram with proper set-up is given below:


Rutherford

Rutherford scattering experiment diagram


The high-energy alpha particles from the alpha-particle emitter are pointed to the gold foil through a slit. The alpha particles were bombarded on the gold foil placed before the movable zinc sulphide fluorescent screen. A fluorescent screen was used for the purpose of detecting alpha particles. 


Observations of  Rutherford Alpha Scattering Experiment

The observations after the successful execution of  the Rutherford scattering experiment were:

  1. Most of the α-particles that were bombarded on the gold foil passed through the foil with no deflection. It indicated that most of the space inside an atom is empty. 

  2. Some of the α-particles were deflected from the gold foil by small angles. This indicated that the positive charge in an atom is not uniformly distributed. This disproved the J. J. Thomson, uniformly distributed positive charges from the plum pudding model. 

  3. A very small amount of α-particles (1-2%) were deflected back from the gold foil at a nearly 180° angle. This indicated that the positive changes are concentrated in a very small volume. The size of the positively charged particles is very small as compared to the total size of the atom.


Rutherford’s Atomic Model

Based on the observations of the scattering experiment, Rutherford proposed the atomic structure of elements. According to Rutherford's atomic model: 

  1. The positively charged particles are concentrated in a very small volume as compared to the total size of the atom. Most of the atom’s mass is also concentrated in this small volume. Rutherford called this small region inside that atom a “Nucleus”.

  2. Rutherford proposed that the negatively charged particles (electrons) are surrounded by the positively charged nucleus. The electrons move around the nucleus in a circular path at a high speed. He named the circular path of electrons “Orbit”.

  3. The dense positively charged nucleus and surrounding negatively charged electrons are held together by a force of attraction called electrostatic forces of attraction. 


Observations

Observations of the Rutherford model


Advantages of the Rutherford’s Model

  • This model discovered the presence of a nucleus in an atom. According to Rutherford, the nucleus is a region in an atom where entire positive charges are concentrated.

  • It correctly postulated that most of the atom’s mass is concentrated in the small region of the nucleus. 

  • Rutherford postulated that the negatively charged electrons revolve around the nucleus in a circular path similar to our solar system. 


Drawbacks of the Rutherford’s Model

  • Rutherford's model failed to explain the stability of an atom. From the calculations, the moving electron will collapse in less than $10^{-8}$ seconds, making the electron highly unstable. Thus, the stability of the atom couldn’t be explained by the Rutherford model.

  • According to Maxwell's theory, charged particles in a circular motion will experience acceleration and emit electromagnetic radiation. So the moving electrons around the nucleus in an atom should emit electromagnetic radiation. The electromagnetic radiation will carry energy from the circular motion of the electron and this will lead to the shrinking of the electron's orbit. This will end the collapsing of the electron with the nucleus. 

  • The Rutherford theory of atomic structure failed to explain the electron arrangement in an atom which was a major drawback of the model.


Conclusion

In the Rutherford alpha particle scattering experiment, alpha particles were bombarded with gold foil. The scattering of alpha particles by gold foil was observed with the fluorescent sheet of zinc sulphide and observations were noted. The groundbreaking observations were noted in the Rutherford atomic model. The Rutherford atomic model was able to explain a lot of theories that the previous Thomson’s plum pudding model failed to explain. Although Rutherford's model explained a lot of theories, it failed to explain the stability and electron arrangement in an atom.

FAQs on Rutherford Scattering Experiment and Atomic Model for JEE

1. What did the Rutherford scattering experiment demonstrate?

The famous Rutherford alpha particle scattering experiment explained the atomic structure of elements. Based on the observations from the scattering experiment, Rutherford introduced his atomic model. In this model, Rutherford explained that most of the space inside an atom is empty. The entire positively charged elements and most of the atomic mass is concentrated in a small volume. Rutherford named this small volume inside an atom as “nucleus”. The negatively charged electrons moved around the nucleus in an optical path called “orbit”.

2. What was the reason to use gold foil in Rutherford's experiment?

For the scattering experiment Rutherford needed a metal sheet that could be as thin as possible and has high malleability. Malleability is a property of metals by which the metal possesses the ability to be hammered or pressed into any shape without breaking. Gold and silver are metals with high malleability. Gold can be easily converted into very thin sheets because of its high malleability and would prevent the absorption of alpha particles. Hence, for the scattering experiment, Rutherford selected gold foil.

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