Rutherford's Model of Atom

Rutherford's Model of the Atom, also known as the Rutherford Model or the Planetary Model, was proposed by Ernest Rutherford in 1911. It revolutionized the understanding of atomic structure and replaced Thomson's Plum Pudding Model.

Rutherford's Scattering Experiment

Rutherford's scattering experiment, also known as the gold foil experiment, was conducted by physicist Ernest Rutherford and his students (Hans Geiger and Ernest Marsden) in 1909. The experiment aimed to investigate the structure of the atom and test the Plum Pudding Model proposed by J.J. Thomson.

Rutherford and his students (Hans Geiger and Ernest Marsden) bombarded very thin gold foil with α–particles. A stream of high energy α–particles from a radioactive source was directed at a thin foil (about 0.00004 cm thick) of gold metal. The thin gold foil had a circular fluorescent zinc sulphide screen around it. Whenever α–particles struck the screen, a tiny flash of light was produced at that point. The results of scattering experiment were quite unexpected.

According to Thomson's Plum Pudding Model of atom, the mass of each gold atom in the foil should have been spread evenly over the entire atom, and α–particles had enough energy to pass directly through such a uniform distribution of mass. It was expected that the particles would slow down and change directions only by a small angles as they passed through the foil.

It was observed that:
  1. Most of the α–particles passed through the gold foil undeflected.
  2. A small fraction of the α–particles was deflected by small angles.
  3. A very few α–particles (∼1 in 20,000) bounced back, that is, were deflected by nearly 180°.

On the basis of the observations, Rutherford drew the following conclusions regarding the structure of atom:
  1. Most of the space in the atom is empty as most of the α–particles passed through the foil undeflected.
  2. A few positively charged α–particles were deflected. The deflection must be due to enormous repulsive force showing that the positive charge of the atom is not spread throughout the atom as Thomson had presumed. The positive charge has to be concentrated in a very small volume that repelled and deflected the positively charged α–particles.
  3. Calculations by Rutherford showed that the volume occupied by the nucleus is negligibly small as compared to the total volume of the atom. The radius of the atom is about 10⁻¹⁰ m, while that of nucleus is 10⁻¹⁵ m. One can appreciate this difference in size by realising that if a cricket ball represents a nucleus, then the radius of atom would be about 5 km.
On the basis of above observations and conclusions, Rutherford proposed the nuclear model of atom in 1911.

Rutherford's Nuclear Model of an Atom

Based on the findings of his scattering experiment, Rutherford proposed a new model of the atom in 1911, known as the nuclear model, which most of the atom's mass and positive charge is concentrated in a tiny, dense nucleus at the center. The nucleus is surrounded by orbiting electrons at a relatively large distance, similar to planets orbiting around the sun. This model explained the deflection of alpha particles observed in the scattering experiment. When alpha particles approached the positively charged nucleus, they experienced strong repulsion, causing them to be deflected or bounce back.
The key features of Rutherford's nuclear model include:
  1. An atom consists of a tiny positively charged nucleus at its contre.
  2. The positive charge of the nucleus is due to protons. The mass of the nucleus, on the other hand, is due to protons and some neutral particles each having mass nearly equal to the mass of proton. This neutral particle, called neutron, was discovered later on by physicist Sir James Chadwick in 1932. Protons and neutrons present in the nucleus are collectively also known as nucleons. The total number of nucleons is termed as mass number (A) of the atom.
  3. The nucleus is surrounded by electrons that move around the nucleus with very high speed in circular paths called orbits. Thus, Rutherford's model of atom resembles the solar system in which the sun plays the role of the nucleus and the planets that of revolving electrons.
  4. The number of electrons in an atom is equal to the number of protons in it. Thus, the total positive charge of the nucleus exactly balances the total negative charge in the atom making it electrically neutral. The number of protons in an atom is called its atomic number (Z).
  5. Electrons and the nucleus are held together by electrostatic forces of attraction.

Defects of Rutherford's Model

While Ernest Rutherford's nuclear model of the atom made significant advancements in understanding atomic structure, it also had several notable limitations or defects:
  • According to Rutherford's model, an atom consists of a positive nucleus with the electrons moving a round it in circular orbits. However James Clerk Maxwell shown that whenever an electron is subjected to acceleration, it emits radiation and loses energy. As a result of this, its orbit should become smaller and smaller and finally it should drop into the nucleus by following a spiral path. This means that atom would collapse and thus Rutherford's model failed to explain stability of atoms.
  • Another drawback is that Rutherford's model said nothing about the electronic structure of the atoms, i.e., how the electrons are distributed around the nucleus and what are the energies of these electrons. Therefore, this model failed to explain the existence of certain definite lines in the hydrogen spectrum.
  • Rutherford does not explain the relationship between atomic mass and atomic number (number of protons). This problem was later solved by James Chadwick by discovering the neutron, the third particle of the atom.

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