In this chapter, you will understand Thomson’s atomic model, also known as plum pudding model of atom and its drawbacks or limitations. After the discovery of sub-atomic particles, such as electron and proton, the scientists were faced several major challenges:
- to understand the stability of atom.
- to compare the behaviour of elements based on their physical and chemical properties.
- to explain the formation of different kinds of molecules by the combination of different atoms.
- to understand the origin and nature of the characteristics of electromagnetic radiation absorbed or emitted by atoms.
Scientists proposed various atomic models to explain the arrangement and distribution of charged particles (electrons and protons) within an atom. While some of these models failed to explain the stability of atoms, two notable models that made significant contributions were:
- J.J. Thomson’s Atomic Model (1904)
- Ernest Rutherford’s Model (1911)
J. J. Thomson’s Atomic Model
After the discovery of electrons and protons within an atom, the next question was to know how these particles are arranged in an atom. In 1898, J. J. Thomson was proposed the first scientific model to explain the internal structure of an atom. This atomic model is also popularly known as plum pudding model of the atom. He proposed his model of an atom before the discovery of the neutron.
J. J. Thomson proposed that an atom consists of a uniform sphere, with a radius of approximately 10-10 m or 10-8 cm in which the positive charge is uniformly distributed over the entire sphere. The negatively charged particles (electrons) are embedded within this sphere in such a way that they create the most stable electrostatic arrangement.
At equilibrium, the total positive charge inside the atom is equal to the total negative charge possessed by the electrons. As a result, the net charge in the system is zero, making every atom is electrically neutral.
Positive charge = negative charge (at equilibrium)
An important feature of Thomson’s atomic model is that the mass of the atom is considered to be uniformly distributed throughout the atom. This model of atom has given many different names, such as plum pudding model, or raisin pudding model or watermelon model. It successfully described the phenomenon of thermionic emission, photoelectric emission, and ionization.
Why Is Thomson’s Atomic Model Called Plum Pudding Model?
J. J. Thomson’s model is called the “plum pudding model” because he compared the atom to a British Christmas cake known as plum pudding. In this model, the atom is visualized as a soft, round pudding that represents the positive charge, and electrons like small plums scattered throughout over the pudding.
According to the plum pudding model, the positive charge and the electrons are mixed together inside the atom, similar to how plums are mixed into a pudding. Hence, the plum pudding model of the atom describes an atom as a sphere of positive charge with electrons embedded within it, much like plums into pudding.
Drawbacks or Limitations of Thomson’s Model of Atom
Although this atomic model was able to explain the overall neutrality of the atom, but over the development of new concepts, this model was discarded because of the following reasons:
- This model failed to explain the origin of the spectral lines in the form of series observed in atomic spectra. For example, hydrogen atom. Niels Bohr’s model and quantum mechanics explained it later.
- It failed to explain the scattering of alpha particles through the large angles in the Ernest Rutherford’s gold foil experiment in 1909.
Thomson’s atomic model was an earlier step in the development of atomic theory because it introduced the concept of the electron. He was awarded by Nobel Prize for physics in 1906, for his theoretical and experimental researches on the conduction of electricity by gases.
