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Thomson’s Atomic Model

Every atom is uniformly positive charged sphere of radius of the order of 10-10 m, in which entire mass is uniformly distributed and negative charged electrons are embedded randomly. The atom as a whole is neutral.
Thomson`s atomic modelThomson's atomic model


Limitations of Thomson’s Atomic Model

  • It could not explain the origin of spectral series of hydrogen and other atoms.
  • It could not explain large angle scattering of α – particles.

Question for Rutherford's Nuclear Model of an Atom
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What limitation of Thomson's atomic model is related to the inability to explain the origin of spectral series of hydrogen and other atoms?
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Rutherford's Nuclear Model of an Atom

  • In 1911, Rutherford, along with Geiger and Marsden, conducted the Alpha Particle scattering experiment.
  • This experiment marked the inception of the 'nuclear model of an atom,' reshaping our understanding of atomic structure.

The Alpha Particle Scattering Experiment

  • They conducted an experiment using a thin gold foil placed in the center of a rotatable detector made of zinc sulfide and a microscope.
  • A beam of 5.5MeV alpha particles from a radioactive source was directed at the foil, collimated by lead bricks.
  • Scattering of alpha particles could be observed by flashes on the screen post-collision.

Alpha Particle Scattering ExperimentAlpha Particle Scattering Experiment

Observations

  • Majority of alpha particles passed through the foil without collisions.
  • Approximately 0.14% of alpha particles scattered by more than 1 degree.
  • Around 1 in 8000 alpha particles deflected by more than 90 degrees.

Conclusions and Arguments

  • Results conflicted with Thomson's plum-pudding model of the atom.
  • Rutherford proposed a concentrated positive charge at the atom's center based on deflection observations.

The Experiment by Rutherford

  • Incident Alpha Particle and Deflection

    When the incident alpha particle approaches the positive mass at the center of the atom closely, it experiences repulsion, leading to deflection. Conversely, if it passes at a considerable distance from this mass, there is no deflection; it simply continues through.

  • Nuclear Model of an Atom

    Rutherford proposed the 'nuclear model of an atom,' where the nucleus holds the entire positive charge and most of the mass. Electrons orbit around the nucleus similar to planets around the sun. He determined the nucleus size to be between 10^-15 and 10^-14 meters.

  • Size Discrepancy and Atom Structure

    According to the Kinetic theory, the atom's size is approximately 10^-10 meters, making it 10,000 to 100,000 times larger than the nucleus suggested by Rutherford. Consequently, electrons should be positioned around 10,000 to 100,000 times the nucleus's size away from it.

  • Empty Space in Atoms

    The experiment highlighted that most of an atom comprises empty space, explaining why a significant number of alpha particles passed through the foil. When alpha particles come close to the nucleus, they get deflected or scattered at wide angles. Since electrons have minimal mass, they do not alter the path of these incident alpha particles.

Alpha Particle Trajectory

  • The path followed by an alpha particle is determined by the impact parameter of the collision. This parameter represents the perpendicular distance of each alpha particle from the center of the nucleus.
  • When all alpha particles in a beam possess identical kinetic energy, their scattering behavior is solely influenced by the impact parameter.
  • Alpha particles with a small impact parameter, i.e., those closer to the nucleus, undergo significant scattering, often at large angles.
  • Conversely, alpha particles with a large impact parameter experience no deflection or scattering whatsoever.
  • Alpha particles approaching with almost zero impact parameter, engaging in a head-on collision with the nucleus, bounce back.
  • During the experimental observations conducted by Rutherford and his team, only a minute fraction of incident alpha particles exhibited rebounding behavior, implying that very few particles collided directly with the nucleus.
  • This observation led to the inference that the mass of an atom is concentrated within an extremely small volume.

Question for Rutherford's Nuclear Model of an Atom
Try yourself:
What did Rutherford propose based on the observations from the Alpha Particle Scattering Experiment?
View Solution

Distance of Closest Approach

ro = 1 / 4π εo . 2Ze2 / Ek

where, Ek = kinetic energy of the cc-particle.

Impact Parameter

The perpendicular distance of the velocity vector of a-particle from the central line of the nucleus, when the particle is far away from the nucleus is called impact parameter.

Impact parameter

Rutherford`s Nuclear Model of an Atom | Physics Class 12 - NEET

where, Z = atomic number of the nucleus, Ek = kinetic energy of the c-particle and θ = angle of scattering.

Limitations of Rutherford Atomic Model

  • About the Stability of Atom: According to Maxwell’s electromagnetic wave theory electron should emit energy in the form of electromagnetic wave during its orbital motion. Therefore. radius of orbit of electron will decrease gradually and ultimately it will fall in the nucleus.
  • About the Line Spectrum: Rutherford atomic model cannot explain atomic line spectrum.
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FAQs on Rutherford's Nuclear Model of an Atom - Physics Class 12 - NEET

1. What are the main differences between Dalton's Atomic Theory, Thomson's Atomic Model, and Rutherford's Atomic Model?
Ans. Dalton's Atomic Theory proposed that atoms were indivisible and indestructible, while Thomson's Atomic Model introduced the concept of the electron and the plum pudding model. Rutherford's Atomic Model, on the other hand, proposed that atoms have a dense nucleus surrounded by electrons.
2. How did Rutherford's Nuclear Model of an Atom contribute to our understanding of atomic structure?
Ans. Rutherford's Nuclear Model of an Atom showed that atoms have a small, dense nucleus at their center, which contains positively charged protons. This model helped to explain how most of the atom's mass is concentrated in the nucleus and how electrons orbit around it.
3. What experimental evidence supported Rutherford's Atomic Model?
Ans. The famous gold foil experiment conducted by Rutherford provided evidence for his atomic model. When alpha particles were shot at a thin gold foil, some were deflected at large angles, indicating the presence of a small, positively charged nucleus in the atom.
4. How did Thomson's Atomic Model differ from Dalton's Atomic Theory?
Ans. Thomson's Atomic Model introduced the concept of the electron, which was not part of Dalton's Atomic Theory. Thomson's model also proposed that electrons were embedded in a positively charged sphere, unlike Dalton's idea of indivisible atoms.
5. What impact did Rutherford's Atomic Model have on the development of modern atomic theory?
Ans. Rutherford's Atomic Model laid the foundation for the development of modern atomic theory by establishing the presence of a nucleus in the atom and the arrangement of electrons around it. This model helped pave the way for further discoveries in subatomic particles and quantum mechanics.
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