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Alpha Particle Scattering and Rutherford Model of Atom - JEE Important Topic

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Last updated date: 13th Jul 2024
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Rutherford Model of Atom: Discovery of Nucleus

An experiment was performed by the scientist Ernst Marsden under the guidance of Rutherford and Hans Geiger for the analysis of structure of atoms by emitting alpha particles from the uranium metals and passing them through the gold foil. This experimental analysis gave birth to the nuclear model of atoms which defines the structure of the atom by the scattering of alpha particles after getting deflected when passed through a foil. This experiment was recognized as the “Rutherford model” and “Gold foil experiment”.


In general terms, this experiment is also acknowledged as Rutherford scattering experiment, alpha particle scattering experiment, Rutherford scattering or Rutherford scattering of alpha particles. Rutherford scattering model depicts that the atoms are made up of a positively charged nucleus that is situated in the centre of an atom and negatively charged electrons that are revolving around the nucleus. However, the Rutherford model was unable to explain the stability of an atom and also didn’t explain the arrangement of electrons inside the atom.


Origin of Alpha Particle Scattering and Rutherford Model of Atom

In 1899, an experiment was performed to study the absorption of radioactive rays by equipping a sheet of thin metal foil. Ernst Rutherford observed two major radiations named alpha radiation and beta radiation as major findings of the above experiment. From this successful attempt, Ernst Rutherford and Hans Geiger studied the structure of an atom with the help of alpha particles scattering by a thin gold foil sheet. The alpha particles were emitted from uranium. In 1911, this experimental investigation was acknowledged as the “Gold foil experiment” or “Rutherford model”.


The Gold Foil experiment is an experimental investigation that was performed by Ernst Marsden under the guidance of Rutherford and Hans Geiger. The research was published in 1911. This experiment became a major asset, which proved that the atom consists of a dense positively charged particle having significant mass, named as a nucleus, with electrons revolving around it.


Gold Foil Experiment and Role of Alpha Particles Scattering

This experimental setup consists of a thin sheet of gold foil, a radioactive source (uranium), a detector having a screen coated with zinc sulphide and a lead screen with slits in itself.


Experimental Setup of Gold Foil Experiment


Experimental Setup of Gold Foil Experiment


In this experiment, a narrow beam of positively charged alpha particles emitted from the uranium metal (radioactive source) was projected to the lead screen. These focused and projected narrow beams of alpha particles passed the lead screen through the slit and stroked the thin sheet of gold foil. After getting deflected from the gold foil, the presence of alpha particles is recorded on the screen of the detector which was coated with zinc sulphide.


Whenever the alpha particles strike the screen of the detector, it generates a fluorescent effect and produces a bombardment effect with a burst of light at the screen. This confirmed the detection of alpha particles. This experiment was observed by a microscope which was placed just behind the backside of the screen of the detector. Based on this observation, various conclusive remarks have been made on the alpha particles scattering and the Rutherford model.


Experimental Effects of Alpha Particles in Rutherford Scattering Experiment

This process recorded various experimental evidence about the structure of the atom which were derived from the deflection of alpha particles.


It was found that the maximum percentage of alpha particles passed through the foil and faced zero deflection throughout their trajectory. This analysis proved that the atom consists of open space in itself. In contrast to the above observation, it was also observed that few alpha particles were scattered by very small angles in their trajectories after getting deflected from the thin sheet of gold foil.


This observation resembles the fact that there must be a particle having mass with the same charge as alpha particles situated inside the atom. When this charged particle interacted with the alpha particles, a repulsive force generated between them and the alpha particles got scattered.


An interesting observation was also recorded that states that a very small number of alpha particles were deflected and returned to their own path. This observation confirmed the presence of a heavy positively charged particle in the atom that was further recognized as the “nucleus”. The nucleus creates a strong repulsive force and makes the alpha particles go back through the same path.


Depending upon the observation, Rutherford gave the structure of the atom by stating that the atom consists of heavy positively charged particles in the centre named the nucleus. Around the nucleus, the alpha particles faced zero deflection which confirmed the presence of negatively charged particles termed as electrons.


Rutherford Atomic Model

In context to the observations derived from the gold foil experiment, Rutherford proposed the atomic model. In this model, Rutherford states that:

  • The nucleus is found to be a specific region of positive charge having a maximum mass of the atom to be concentrated in it.

  • It was also stated that the negatively charged particles revolve around the nucleus at a very high speed in circular orbits.

  • In an atom, electrons and the nucleus are bound together due to the strong electrostatic force between them.

  • Some of the alpha particles suffered a deflection of 90 degree while some of them suffered a collision of 180 degree after striking with the gold foil which confirms the presence of big particles having the same charge as of alpha particles inside the atom.

  • Few of the alpha particles didn’t face any obstruction after being struck with the gold foil.


Limitation of Rutherford Atomic Model

  • According to the Rutherford atomic model, it was stated that electrons revolve around the nucleus in a fixed path called orbits. However, this previous statement contradicts Maxwell’s theory. In Maxwell’s theory, the motion of electrons around the nucleus emits electromagnetic radiation which withdraws energy from the motion of charged particles. According to it, electrons might collapse in the nucleus.  In this case, Rutherford's model didn’t agree with Maxwell's theory. 

  • Rutherford's atomic model didn’t explain the concept of the stability of an atom and the positioning of electrons inside the structure of the atom.


Conclusion

This article concluded the information regarding alpha particle scattering and Rutherford model of the atom. It analyses the structure of the atom by giving insight into the Rutherford scattering experiment. It shares the glimpses of the gold foil experiment which was performed by Ernst Marsden under the guidance of Rutherford and Hans Geiger.


This experiment discusses the evidence of the presence of a nucleus and electrons inside the atom with the help of alpha particles scattering. The major findings of this experiment confirm that the nucleus is a positively charged particle placed in the centre of the atom and a negatively charged electron revolves around it.

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FAQs on Alpha Particle Scattering and Rutherford Model of Atom - JEE Important Topic

1. What is the significance of the gold foil experiment?

 The significance of this experiment titled as “Gold foil Experiment'' was the determination and analysis of the structure of the atom. Based on this observation, it was concluded that an atom consists of three particles i.e. nucleus, electrons and protons. 


It also depicted that negatively charged particles called electrons revolve around the positively charged particle called the nucleus in a definite circular path called orbits inside the atom. It also states that the maximum mass of the atom is concentrated in the nucleus.

2. What is the role of the screen of the detector coated with zinc sulphide used in the gold foil experiment?  

The screen of the detector used in the gold foil experiment was coated with zinc sulphide. It is used as a counter of the alpha particles as it creates a fluorescent effect with the flashes of light as many times as the alpha particles strike it. It also acted as a collector of the alpha particles which were emitted from the uranium metal (source) and passed through the slit and the gold foil.  The gold foil was surrounded by the detector for the detection of alpha particles.