J.J. Thomson Atomic Model | Postulates, Limitations

J.J.Thomson Atomic Model: J.J. Thomson proposed that an atom consists of a uniform sphere in which positive charge is uniformly distributed. The electrons are embedded into it in such a way as to give the most stable electrostatic arrangement. The sphere’s radius is of the order of 10-10 m, which is equal to the size of the atom.

jj thomson atomic model

This model was much like pudding or cake (of positive charge) with raisins (electrons) embedded into it. Therefore, this model is also known as the Raisin pudding model.

This model was also compared with the watermelon model of positive charge in which seeds (electrons) are embedded. Therefore, this model is given different names, such as raisin pudding, plum pudding, or watermelon model.

Postulates of J.J. Thomson’s Atomic Model

Postulates of J.J. Thomson's Atomic Model

Here are some postulates of J.J.Thomson Atomic Model

Postulate-1:

An atom consists of a positively charged sphere with electrons embedded in it.

Postulate-2:

An atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude.

Thomson atomic model is compared to watermelon. Where he considered:

  • Watermelon seeds as negatively charged particles.
  • The red part of the watermelon as positively charged.

History

Before discovering subatomic particles, John Dalton came up with Dalton’s atomic theory, where he suggested that atoms are indivisible particles. It explained atoms could not be broken down into other smaller particles. However, the discovery of subatomic particles disapproved the postulates proposed in Dalton’s Atomic Theory.

The discovery of subatomic particles led to the search of how the subatomic particles are arranged in an atom.

  • J.J. Thomson was the first and one of the many scientists who proposed models for the structure of an atom.
  • J.J. Thomson discovered negatively charged particles by cathode ray tube experiment in the year 1897.

The particles were named electrons. J.J Thomson believed electrons to be two thousand times lighter than a proton. He assumed that an atom is composed of a cloud of negative charge in a sphere of positive charges. However, J.J Thomson and Rutherford first demonstrated the ionization of air in x rays.

Limitations of J.J. Thomson Atomic Model

Limitations of J.J. Thomson Atomic Model
  1. It failed to explain the stability of an atom because his model of atom failed to explain how a positive charge holds the negatively charged electrons in an atom. Therefore, This theory also failed to account for the position of the nucleus in an atom.
  2. Thomson’s model failed to explain the scattering of alpha particles by thin metal foils.
  3. No experimental evidence in its support.

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