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Thomson’s Atomic Model

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Thomson’s Atomic Model is one of the fundamental models of the atom that tries to explain the working and structure of the atom. this model was proposed by famous Scientist JJ Thomson in 1904. Thomson during his cathode ray experiment proved the existence of a negatively charged particle called electron that exists inside the atom. And after complaining about his result he prosed the Thomson atomic model also called Pulm Pudding Model. 

In this article, we will learn more about, Thomson Atomic Model, its postulate, and its limitations in detail. 

Thomson’s Atomic Model

Thomson’s Atomic Model was proposed by J.J. Thomson in 1904, following his discovery of the electron in 1897 after his famous cathode ray experiment. The model is also known as the Plum Pudding Model. In this model, Thomson proposed that atoms are composed of a positively charged sphere with negatively charged electrons embedded within it. The positive charge of the sphere was thought to be uniformly distributed to balance out the negative charge of the electrons. The electrons were held in place by electrostatic attraction to the positive sphere.

The model explained certain experimental observations of that time that atoms were electrically neutral and that cathode rays were negatively charged. The atomic model was later discarded as it was unable to explain the various results concluded by various scientists after their experiments such as it was unable to explain the existence of the nucleus whose existence was confirmed by Rutherford in his experiment.

Plum Pudding Model

Thomson Atomic Model is also called the Plum Pudding model as it represents the Plum Pudding. According to Thomson, an atom represents a sphere of radius of magnitude in Angstrom. (10-10). In this sphere, the positive charge is spread uniformly and the negative charge is embedded in this positive charge due to electrostatic attraction between the charge. The magnitude of the positive and the negative charge in the sphere is equal according to Plum Pudding Model.

Observations of Thomson’s Atomic Model

Thomson’s Atomic Model, also called the Plum Pudding Model states that an atom can be compared to a plum pudding where electrons are like dry fruits in a sphere of positive charge, representing the pudding. We can also compare this with a watermelon where the positive charge is comparable to the edible part of the watermelon, and electrons are like seeds that are embedded in that sphere.

The diagram explaining the Plum Pudding Model or the Thomson Atomic model is added below which shows that positive charge is spread throughout the atom and negative charge is embedded in the atom maintaining the electrical neutrality of the atom.

Thomson's Atomic Model

 

Postulates of Thomson’s Atomic Model

Various Postulates of Thomson’s Atomic Model are mentioned below:

  • An atom consists of a positively charged sphere or cloud in which negatively charged electrons are embedded. The positive charge of the sphere is uniformly distributed to balance the negative charge of the electrons.
  • An atom is an electrically neutral quantity and the total positive charge of the atom is balanced by the total negative charge in the atom.

Thomson’s model is also called Plum Pudding Model or Watermelon Model.

Limitations of Thomson’s Atomic Model

Thomson’s atomic model had several limitations that were eventually overcome by further research and the experiments of various scientists later on. Some of the limitations of Thomson’s Atomic Model are:

  • This model did not account for the presence of positively charged particles in the atom, which were later identified as protons by Ernest Rutherford. The model assumed that the positive charge was distributed uniformly throughout the atom.
  • The model could not explain the results of the gold foil experiment conducted by Rutherford, which showed that most of the mass and positive charge of an atom is concentrated in a small, dense nucleus at the centre of the atom.
  • The model could not explain the phenomenon of atomic spectra, which is the emission and absorption of specific wavelengths of light by atoms.
  • The model did not explain the presence of Isotopes.

Despite these limitations, Thomson’s Atomic Model is an important step in the development of Atomic Theory and only after this theory, there came various theories that provide a better understanding of Atoms.

Background of Thomson’s Atomic Model

The first scientific discussion about the structure of atoms was started by famous scientist JJ Thompsom in 1897. He stated that atoms are spherical shape objects in which electrons are embedded in a jelly-like substance and the jelly-like substance is called protons. Initially, scientists are of the view that atoms are indivisible and they did not believe the explanation of Thomson stating that atoms are made up of smaller particles. This was proved later by his cathode ray experiment which proved that atoms are made up of negatively charged particles called Electrons.

Thomson Atomic Model is not the most accurate model to explain the structure of an atom as it has various drawbacks but it was the first model to explain the basic structure of an atom that atom as a whole is neutral and it consists of positive and negative charge in equal magnitude. After Thomson, various scientists proposed the atomic structure which was further enhanced to get the idea of the atom which prevails today.

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FAQs on Thomson’s Atomic Model

Q1: What is Thomson’s Atomic Model?

Answer:

Thomson’s Atomic Model was the first model to explain the basic structure of the atom. It was given by famous scientist JJ Thomson after the discovery of electrons from his cathode ray experiment. He claimed that atoms are made up of electrons and protons and the electrons are embedded in the continuous spread of protons as the pulm is spread in the pudding. Hence this model is also called the Pulm-Pudding model, according to him atom is electrically neutral and the magnitude of positive and negative charges are equal.

Q2: Which Model best explains Atom’s Neutrality?

Answer:

Thomson’s Atomic Model was the one which first explains that as a whole an atom is an electrically neutral entity. He states that the magnitude of the negative charge (electrons) and the magnitude of the positive charge (protons) are equal in an atom.

Q3: What are the major Drawbacks of Thomson’s Model?

Answer:

Various drawbacks of Thomson’s Atomic Model are,

  • It fails to explain the existence of the neutrons that were discovered by Chadwick.
  • It fails to explain the finding of Rutherfor’s Gold Foil experiment.
  • It does not explain the existence of isoelectric species, etc.

Q4: What are the Important Features of Thomson Atomic Model?

Answer:

The important features of the Thomson Atomic model are,

  • It was the first model to think that atom is like a sphere.
  • It successfully explains the neutrality of the atom.
  • It explains that the magnitude of positive and negative charges in an atom is equal.

Last Updated : 19 Dec, 2023
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