Drawbacks of Rutherford Atomic Model, Rutherford Atomic Model, Question (For CBSE, ICSE, IAS, NET, NRA 2022)

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Drawbacks of Rutherford Atomic Model

  • Rutherford atomic model was the first step in the evolution of the modern atomic model. Ernest Rutherford was a keen scientist who worked to understand the distribution of electrons in an atom. He performed an experiment using alpha particles and gold foil and made the following observations:
    • Most of the alpha particles passed straight through the gold foil.
    • There was a deflection of the alpha particles by a small angle.
    • Very small number of alpha particles rebounded.
Rutherford Experiment

Rutherford՚s Experiment

  • From his experiment, he came to following conclusions:
    • Most of the space in an atom is empty.
    • The space occupied by the positive charges is very small.
    • The positive charges and mass of the atom were concentrated in a very small volume within an atom.
  • From these conclusions, he calculated that the radius of the nucleus is around 105 times less than that of the atom.

Rutherford Atomic Model

Rutherford developed a nuclear model of atom on the basis of his experiment and observations. The Rutherford atomic model has the following features:

  • The centre of an atom is called nucleus. It is positively charged and almost all mass of the atom resides in it.
  • Electrons spin around the nucleus in a circular path.
  • Comparatively, the size of the nucleus is smaller than the size of the atom.
Rutherford Atomic Model

Rutherford Atomic Model

Rutherford՚s Scattering Experiment or Gold Foil Experiment

  • In 1911, A physicist from New Zealand Ernest Rutherford performed scattering experiment to study atoms.
  • The apparatus used in the experiment is shown in the figure below-
  • In 1899, Rutherford discovered that some elements give off positively charged particles, he named them particles. Thus, particles are positively charged particles. In his scattering experiment to study atoms, he produced u -particles by using a radioactive polonium (or radium) . Which he kept in a lead box (as shown in the figure above) . A narrow beam of these particles come out from a deep hole in the lead block. The beam is collimated by a lead slit and directed at a thin gold foil (approximately 100 nm) .
  • The a-particles are detected by the light they produce when they collide with the coated screen. The entire apparatus is enclosed in a vacuum chamber, so that deflections of a-particles are not influenced by the collision with gaseous molecules. A moving microscope is used for observation of direction of light rays.

During Rutherford՚s u -particle scattering experiment following observations were made —

  • Most of the a-particles passed through the gold foil.
  • Some of the u -particles were deflected by the foil making small angles.
  • Surprisingly one out of every 12000 particles appeared to rebound.
  • Rutherford said “this result was almost as incredible as if you fire a inch shell at a piece of tissue paper and it comes back and hits you.”

Conclusions made by Rutherford through his u -particle scattering experiment-

  • Most of the space inside the atom is empty as most of the a-particles passed through the gold foil without getting deflected.
  • Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
  • A very small fraction of particles was deflected by 1800, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom. By this he concluded that the radius of the nucleus is about times less than the radius of the atom.

Drawbacks of Rutherford Atomic Model

  • As before, Rutherford atomic model was also challenged and questioned by many. Rutherford atomic model failed to explain about the stability of electrons in a circular path.
  • As per Rutherford՚s model, electrons revolve around the nucleus in a circular path. But particles that are in motion on a circular path would undergo acceleration, and acceleration causes radiation of energy by charged particles.
  • Eventually, electrons should lose energy and fall into the nucleus. And this points to the instability of atom. But this is not possible because atoms are stable. Hence, Rutherford failed to give an explanation on account of this.

Question

What Are the Limitations of Rutherford Atomic Model?

Answer:

Limitations of Rutherford Atomic Model

Although the Rutherford atomic model was based on experimental observations it failed to explain certain things. Rutherford proposed that the electrons revolve around the nucleus in fixed paths called orbits.

Why Did Rutherford Use Alpha Rays?

Answer:

In 1899, studying of the radioactive element uranium, Ernest Rutherford discovered that there were two types of emissions measurable by his equipment. Alpha particles are actually the two-proton nuclei of helium atoms discharged during the nuclear decay of unstable elements like uranium.

Why Did Rutherford Use Gold?

Answer:

Rutherford used gold for his scattering experiment because gold is the most malleable metal and he wanted the thinnest layer as possible. The gold sheet used was around 1000 atoms thick. Therefore, Rutherford selected a Gold foil in his alpha scattering experiment.

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