Thomson Atomic Model and their Limitations

Atomic model:

The discoveries of the subatomic particles lead to the drive of curiosity on the arrangement of the atomic particles like electrons, protons and neutrons in atoms. Sir Joseph John Thomson was the first-ever person proposing the model for atomic structure to this world of science. After the discovery of negatively charged particles  or electrons’ by cathode ray tube experiment in the year 1897 and proposed the theoretical atomic molecular structure in the year 1898. Demonstration of ‘Ionisation of air’ in X-rays was done by Sir JJ Thomson and Sir Ernest Rutherford.

Thomson’s Atomic model structure-

Thomson assumed that an atom is made up of  positively charged spheres  in which electrons are embedded. In the year 1898, the concept of the nucleus did not exist. Along with the theories available by them, he proposed that “an atom possesses a spherical shape (approximately of around10-10m) where the positive charges are distributed uniformly.” The electrons were arranged to give the maximum possible electrostatic stable configuration.

The postulates of the atomic model are as follows-

Postulate: I

An atom resembles a well-charged circle with electrons in space.

Postulate: II

Negative and positive charge particles are equal in size and therefore, the atom has zero total charge and as a result the atom is neutral.

Postulate: III

Thomson’s atomic model resembles a round plum pudding and a watermelon. It looks like plum pudding because the electrons in the model look like dried fruit confined in a well-charged area like a round plum pudding.

The model has also been visualised as a watermelon because the red edible part of a watermelon was similar to that of the sphere having a positive charge and the seeds filling the watermelon seemed similar to the electrons located into the sphere. Due to this visualisation of an atomic model to a watermelon and puddings made during Christmas were similar. It is otherwise known as  “plum pudding”, “raisin pudding” or a “watermelon”.

The properties of the postulates were as follows-

• Atoms are neutrally charged bodies
• Electrons are present in atoms and are negatively charged particles

Drawbacks of Thomson’s Atomic Model-

Despite claiming the atoms were electrically neutral, the experiment that was performed by other scientists could not explain the differences in the outputs and the causes. The major drawbacks brought into notice due to which this concept was forced to be expelled out are as follows-

• Thomson’s atomic model could not explain how the positive charged body holds on the electrons situated inside the atom
•  It also fails to explain the stability that an atom possesses
• The concept of the nucleus was not present in this theory
• Rutherford’s gold foil experiment of scattering α-particles showed that most of the space present in an atom is vacant which contradicts the statement of Thomson’s atomic model

Moreover, this model was not sufficient to explain the structure of an atom.

Conclusion

Despite being an unsuccessful contribution to the world of chemistry, it provided the basics to further several experiments and research and helped other scientists to a significant revolutionary invention. Sir JJ Thomson also won the Nobel Prize for Physics in 1906 for his remarkable contribution to the conduction of electricity by means of gases.

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