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explain Thomson's model of an atom...

Thomson atomic model was proposed by William Thomson in the year 1900. This model explained the description of an inner structure of the atom theoretically. It was strongly supported by Sir Joseph Thomson, who had discovered the electron earlier.

During cathode ray tube experiment, a negatively charged particle was discovered by J.J. Thomson. This experiment took place in the year 1897. Cathode ray tube is a vacuum tube. The negative particle was called an electron.

Thomson assumed that an electron is two thousand times lighter than a proton and believed that an atom is made up of thousands of electrons. In this atomic structure model, he considered atoms surrounded by a cloud having positive as well as negative charges. The demonstration of the ionization of air by X-ray was also done by him together with Rutherford. They were the first to demonstrate it. Thomson’s model of an atom is similar to a plum pudding.

 

Thomson Model of Atom
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Thomson model:

According to this model, an atom can be considered as a sphere of uniformly distributed positive charge in which there are electrons distributed symmetrically.

The electrons must be held by the positive charges by electrostatic forces. The mutual repulsions between the electrons are balanced by the force of attraction towards the centre of the sphere.

In an atom with a single electron like the hydrogen atom, the electron must be situated at the centre of the positive sphere.

In an atom with two electrons, like the helium atom, the two electrons must be symmetrically situated on opposite sides of the centre at a distance equal to half the radius of the positive sphere.

In the three-electron system, the electrons should be at the corners of a symmetrically placed equilateral triangle, the side of which is equal to the radius of the sphere.

Proceeding in this manner, Thomson could explain the arrangement of electrons ranging from 1 to 100 inside the positive sphere.

This model was also called 'the plum pudding model'. The electrons are like plums in a pudding (positive charge). The seeds in a watermelon fruit can be given as a parallel comparison.

With this model he tried to explain the observed spectra of elements.

According to him, the vibrations of electrons would give rise to electromagnetic energy radiated. He could explain with some success the process like chemical reaction, radioactivity etc.

Drawback of the Ruthorford 's model:

The model is applicable for one electron system.

According to Rutherford as the electron revolves around the nucleus it liberate energy regularly so when the electron loses all its energy it should fall in the nucleus which was impossible.

The model does not give the objection raised by Clerk Maxwell related to continuous emission of energy by electron.

The Rutherford?s model of atom does not say anything about the arrangement of electrons in an atom.

Isotope:

Isotopes are defined as atoms having the same atomic number, but different mass numbers. These atoms contain an equal number of protons and electrons, but a different number of neutrons.
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he said that it can be like a watermelon too
 
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Hi,
The main features of Thomson’s model of atom are:
  • Atom consists of a positively charged sphere
  • Electrons are embedded in atoms
  • Negative and positive charges are equal in magnitude
  • Atom as a whole is electrically neutral
 
The following diagram illustrates a pictorial representation of the Thomson’s model of atom.


 
This model was explained with the help of a watermelon wherein the red edible part resembles positive charge in the atom and the black seeds resemble the electrons embedded in positive sphere.
This model is also known as plum pudding model
 
 

 
 
 
 
 
 
 
 
 
 
 
The limitation of this model is that it was not able to explain the results of the experiments obtained by other scientists like Ernest Rutherford.
 
Hope, this answer is useful to you.
Best wishes!
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