Thomson's Atomic Model

Introduction – Atomic model

The electron was detected by J.J. Thomson in the year 1897. Often recognized as the presence of protons as was the fact that atoms were in charge of being neutral.


Since the intact atom had no charge and the electron and proton had opposite charges, the next step after subatomic particles were discovered was to determine how these particles were arranged in the atom. 


Because of the incredibly small size of the atom this is a difficult task. Scientists therefore set out to devise a model of what they thought the atom might look like.

 The goal of each atomic model was to present all the experimental evidence of atoms in the simplest way possible


Thomson proposed the atom should be composed of electrons surrounded by a broth of positive charge to counteract the negative charges of the electrons. Which led to plum pudding model.


Plum Pudding Model

J. J. Thomson, who invented the electron in the year 1897, suggested the atom's plum pudding model in 1904 before the atomic nucleus was found, to include the electron in the atomic model.

In this model, the atom consists of electrons (which Thomson called them "corpuscles") surrounded by a soup of positive charge to counteract the negative charges of the electrons, like "plums" charged negatively surrounded by "pudding" charged positively.


The electrons were assumed to be positioned in revolving circles throughout the atom. The atom was also often represented in this model as having a "cloud" of positive charge.


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Thomson with this model abandoned his earlier hypothesis of the "nebular atom," in which the atom was composed of immaterial vortices. Now, at least part of the atom was to consist of the particulate negative corpuscles of Thomson, although the remainder of the positively charged part of the atom remained somewhat nebulous and undefined.


This model is also called as watermelon model of atom because it resembles a spherical plum pudding as well as a watermelon.


The model was also compared with a watermelon because the red edible part of a watermelon was compared to the positively charged sphere and the black seeds filling the watermelon looked similar to the electrons inside the sphere.


Thomson Proposed That

  1. An atom consists of a positively charged sphere, and it embeds the electrons.

  2. In magnitude the negative and the positive charges are equal. So, the whole atom is electrically neutral. Although Thomson's model explained that atoms are electrically neutral, this model could not explain the results of experiments conducted by other scientists.


Limitations of JJ Thomson Model of Atom

Thomson anticipated that: 

An atom consists of a positively charged sphere, and it embeds the electrons. In magnitude the negative and the positive charges are equal. So, the whole atom is electrically neutral.


Thomson's atom model fails to explain Rutherford's α-particle scattering experiment in which most of the fast moving α - particles went through the gold foil straight away.

Only some of the α - particles were deflected by small angles through the foil. That clearly defined atom has much empty space and positive charge within the atom is concentrated in a very small volume.


Therefore, these were the drawbacks of Thomson model of atom which failed to explain atom's stability and scattering experiment of Rutherford.

FAQ (Frequently Asked Questions)

1. What was JJ Thomson's Atomic Model?

J J.J. Experiments with cathode ray tubes by Thomson showed that all the atoms contain tiny subatomic particles or electrons that are negatively charged. Thomson suggested the atom's plum pudding model, which had negatively charged electrons trapped in a "soup" filled with positive effect.


2. Why was JJ Thomson's Model Wrong?

At the time, Thomson's model was correct, because it explained everything scientists already understood about the atom. Thomson 's model was dismissed by the Japanese physicist Hantaro Nagaoka. He said a massive nucleus was in the atom. The electrons, like the rings revolving around Saturn, revolved around the nucleus.

3. Explain JJ Thomson's Contribution to The Atomic Theory?

In the year 1897 J.J. Thomson discovered the electron by playing with a tube that was Crookes, or cathode ray. He had shown that the cathode rays were charged negatively. Additionally, he also examined positively charged neon gas particles.

4. Why is The Plum Pudding Model Wrong?

In 1911, Rutherford proved that the Thomson hypothesis was "wrong": there was no uniform distribution of both positive and negative particles. Rutherford has shown the atom has a small, massive, positively charged nucleus in it. He has also confirmed with Nagaoka that the electrons move outside the nucleus in circular orbits.