Thomson Atomic model: Definition, Experiments, and Limitation

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What is Thomson Atomic Model?

Thomson atomic model was proposed by William Thomson in the year 1900. This model described the inner structure of the atom theoretically.

This theory was strongly supported by Sir Joseph Thomson, who had discovered the electron initially.

Thomson presumed that an electron is two thousand times lighter as compared to proton and thought that an atom is made up of thousands of electrons.

In this atomic structure model, he stated that the atoms are fenced by a cloud having positive as well as negative charges.

Thomson atomic model of an atom is reffered to as a plum pudding. However, at that time the atomic nucleus was yet to be discovered. So, Thomson projected a model on the basis of known properties of atom that was available at that time.

The known properties are mentioned below:

• Normally, atoms are neutrally charged particles.

• Negatively charged particles named as electrons are present in an atom.

Thomson Pudding Atomic Model

Thomson projected that the shape of an atom is similar to that of a sphere having a radius of 10-10 m.

The positively charged particles are uniformly scattered with electrons arranged in such a way that the atom is thus electrostatically stable.

Thomson atomic model was also named as the plum pudding model or the watermelon model.

The electrons present in an atom resembled the seeds that are embedded in a watermelon whereas the watermelon’s red mass signified the positive charge distribution in atom.

Thomson Atomic model, 1 JJ Thomson Atomic model, JJ Thomson model, Thomson model

Postulates of Thomson Atomic Model

An atom generally contains a positively charged sphere with electrons that are embedded in it.

An atom as an entire particle is electrically neutral because the negative and positive charges present are equal in magnitude.

Thomson stated that the model of an atom is similar to the structure of watermelon.

Where he considered:

• Watermelon seeds are represented as negatively charged particles.

• The red part of the watermelon is represented as positively charged.

• According to the postulates of Thomson’s atomic model, an atom is similar to a sphere of positive charge with electrons or negatively charged particles that are present inside the sphere.

• The positive and negative charges present in atom is equal in magnitude.So, an atom has no charge and is considered to be electrically neutral.

• Thomson’s atomic model is similar to a structure of a spherical plum pudding and a watermelon.

Limitations of Thomson Atomic Model

• Thomson was unable to explain the stability of an atom because his model of atom failed to describe how a positive charge particles holds the negatively charged electrons in an atom. Consequently, Thomson’s theory also failed to justify the position of the nucleus in an atom.

• Thomson’s model was unable to explain the scattering of alpha particles by thin metal foils that was explained by Rutherford.

• Even though Thomson’s model was not a precise model to account for the atomic structure, it still proved to be the base for the expansion of other atomic models. The study of the atom and its structure has flagged a way for several inventions that have played an important role in the development of humankind.

Thomson Atomic model Conclusion

Even though Thomson’s atomic model was not precise and had a few drawbacks, it still provided the base for several other atomic structure models that were later discovered. It is thus one of the foundation models that led to important and revolutionary discoveries.

Thomson Atomic model Citations


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