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CBSE Class 9 Chemistry Notes for Chapter4 Structure of Atoms

J.J Thomson came up with a model to explain the structure of atom. According to this model, the atom consists of a positively charged sphere in which the negatively charged particles i.e. electrons are embedded. The positive charges and negative charges are equal in magnitude which makes the atom neutral.

Rutherford conducted an alpha particle scattering experiment using a gold foil. α- particles are doubly charged helium ions. In the experiment, fast moving α- particles are made to fall on a thin gold foil. Fast moving α- particles have high energy. It was expected these particles would get deflected due to the negative charged particles in the atom.
Observations made after the experiment:

  • Most of the fast moving α- particles passed straight through the gold foil and showed no deflection.
  • Some of the α- particles were deflected by the foil be small angles.
  • Some particles appeared to rebound.


  • Most the space inside the atom is empty.
  • The positive charge of the atom occupies very little space.
  • A very small fraction of α- particles rebounded indicating that all the positive charge and mass of the atom is concentrated in a very small volume within an atom.

Based on the above conclusions, Rutherford put forward his model having the following features:

  • There is a positively charged centre in an atom called the nucleus which contains nearly all the mass of an atom.
  • The electrons revolve around the nucleus in well-defined orbits.
  • The size of the nucleus is very small as compared to the size of the atom.

Drawbacks of Rutherford’s model:
Any particle moving in a circular orbit would undergo acceleration, lose energy and eventually fall into the nucleus. This would result in the atom being highly unstable. Therefore, this model could not explain the stability of atom.

This model states that only certain orbits of electrons are allowed inside the atom. While revolving in these orbits, the electrons do not lose energy. These orbits are called energy levels and are represented by the letters K, L, M, N… or 1,2,3,4…
NEUTRONS: A neutron is a sub-atomic particle present in the nucleus. It has no charge and its mass is nearly equal to that of a proton.

Rules for distributing electrons into different orbits:

  • The maximum number of electrons present in a shell is 2n2 where n is the energy level.
  • The maximum number of electrons in the outermost orbit is 8.
  • Electrons are not accommodated in a shell unless the inner shells are filled.

The number of protons present in the nucleus of an atom is called the atomic number of the element. It is denoted by ‘Z’.

Protons and neutrons together are known as nucleons. The number of neutrons and protons i.e. nucleons present in the nucleus is called the mass number.

Isotopes are the atoms of an element having the same atomic number but different mass numbers.

Atoms of different elements with different atomic numbers having same mass number are called isobars.

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