Structure of Atom

Thomson’s Model of an Atom

All of us have seen a plum pudding and a watermelon in our daily life. Do you know one of the early models of an atom has been compared to a plum pudding, raisin pudding, and even a watermelon? The model we are talking about is the Thomson’s atomic model. Any idea why it has been given such names? Let us find out more about Thomson’s atomic model or Thomson’s Model of an Atom.

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History

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

The discovery of subatomic particles led to the search 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. J.J Thomson and Rutherford first demonstrated the ionization of air in x rays.

Learn about the concept of an Atomic number here in detail.

Thomson Model of an atom

The description of Thomson’s atomic model is one of the many scientific models of the atom. It was proposed by J.J Thomson in the year 1904 just after the discovery of electrons. However, at that time the atomic nucleus was yet to be discovered. So, he proposed a model on the basis of known properties available at that time. The known properties are:

Thomson’s Atomic Model- Postulates

  • According to the postulates of Thomson’s atomic model, an atom resembles a sphere of positive charge with electrons (negatively charged particles) present inside the sphere.
  • The positive and negative charge is equal in magnitude and therefore an atom has no charge as a whole and is electrically neutral.
  • Thomson’s atomic model resembles a spherical plum pudding as well as a watermelon. It resembles a plum pudding because the electrons in the model look like the dry fruits embedded in a sphere of positive charge just like a spherical plum pudding. The model has also been compared to a watermelon because the red edible part of a watermelon was compared to the sphere having a positive charge and the black seeds filling the watermelon looked similar to the electrons inside the sphere.
Thomson's atomic model

Thomson’s Atomic Model
               (Source Credit: Wikipedia)

Limitations of Thomson’s Atomic Model

  • Thomson’s atomic model failed to explain how the positive charge holds on the electrons inside the atom. It also failed to explain an atom’s stability.
  • The theory did not mention anything about the nucleus of an atom.
  • It was unable to explain the scattering experiment of Rutherford.

Learn about the Rutherford’s Atomic Model in detail here.

 

Conclusion

Even though Thomson’s atomic model was inaccurate and had a few drawbacks, it provided the base for several other atomic structure models afterward. It is one of the foundation models that led to significant and revolutionary inventions later.

Solved Questions for You

Question: Who gave the first model of an atom?

  1. Ernest Rutherford
  2. J.J Thomson
  3. Eugen Goldstein
  4. Neils Bohr

Solution: The answer is 2 (J.J. Thomson)

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