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Class 9, Science, Chapter-4, Lecture-1, Structure of Atom (Notes)

Thomson’s Model of an Atom:

By Cathode ray experiment Thomson discovered the presence of electrons in atom and proposed that,

  1. An atom consists of a positively charged sphere and the electrons are embedded in it.
  2. The negative and positive charges are equal in magnitude. So, the atoms as a whole is electrically neutral.

Limitations of Thomson’s Model:

  1. Electrically neutral behaviour of atom was not explained well using this model. 
  2. The further experiments deviated from the facts that were considered by J.J Thomson

Rutherford’s α-particle Scattering Experiment:

Procedure:

  1. Fast moving alpha particles (doubly charged Helium ions) were made to fall on a thin gold foil (about 1000 atoms thick).
  2. Photo sensitive screen was placed encircling the gold foil in such a way that the alpha particles may hit the screen after passing through the foil or after rebounding from the foil.
  3. The scintillations produced due to hitting of alpha particles on the photographic plate were observed.

Observations:

  1. Most of the alpha particles passed straight through the gold foil.
  2. Some of the alpha particles deviated by small angles.
  3. About one out of every 20000 alpha particles deviated by more than 45O
  4. About one out of every 12000 alpha particles rebounded.

Conclusion:

  1. Most of the space inside the atom is empty.
  2. Positive charge of the atom occupies very little space.
  3. All the positive charge and mass of the atoms were concentrated in of volume of atom. (Radius of nucleus is times the radius of an atom)

Rutherford’s Model of an Atom:

Based on the observations of Gold Foil Experiment, Rutherford proposed that,

  1. There is a positively charged center in an atom called the nucleus which bears 99.9 % of the mass of the atom.
  2. The electrons revolve around the nucleus in well-defined orbits.
  3. The size of the nucleus is very small as compared the size of the atom.

Drawbacks of Rutherford’s Model:

As per the model, electrons revolve around the nucleus of an atom.
A revolving charged particles tends to lose energy by radiating energy due to acceleration.
Revolving electron must fall into the nucleus after losing all its energy.
This will make the atom unstable. But the existing atoms are stable.
Rutherford’s model could not explain this contradiction.

Bohr’s Model of an Atom:

To explain stability of atoms, Neil Bohr proposed that,

  1. The electrons revolve around the nucleus in well-defined discrete shells having fixed energy level.
  2. The electrons do not radiate energy while revolving in discrete shells called energy levels.

Subatomic Particles:

 ElectronProtonNeutron
DiscoveredJ J Thomson, 1897E Rutherford, 1919J Chadwick, 1932
Charge$${\rm{ -  1}}{\rm{.6 \times 1}}{{\rm{0}}^{{\rm{ - 19}}}}{\rm{ C}}$$${\rm{ +  1}}{\rm{.6 \times 1}}{{\rm{0}}^{{\rm{ - 19}}}}{\rm{ C}}$Neutral
Absolute Mass$${\rm{9}}{\rm{.1 \times 1}}{{\rm{0}}^{{\rm{ - 31}}}}{\rm{ kg}}$$$${\rm{1}}{\rm{.673 \times 1}}{{\rm{0}}^{{\rm{ - 27}}}}{\rm{ kg}}$$$${\rm{1}}{\rm{.674 \times 1}}{{\rm{0}}^{{\rm{ - 27}}}}{\rm{ kg}}$$
Relative Mass$${{\rm{1}} \over {{\rm{1836}}}}{\rm{ u}}$$$${\rm{1 u}}$$$${\rm{1}}{\rm{.001 u}}$$