Thomson’s Model of an Atom (Plum Pudding Model)


Thomson’s model of an atom is also called the plum pudding model. The plum pudding model is one of the several historical scientific models of the atom. First proposed by J.J. Thomson in 1903 soon after the discovery of the electron but before the discovery of the atomic nucleus.

Thomson's Model of an Atom


Thomson’s atomic model  is shown in the above figure. The colored area in the watermelon contains all the positive charge in the atom. The negatively charged electrons are spread throughout the positive charge. The total negative charge of electrons is equal to the total positive charge of the watermelon. These equal and opposite charges balance each other due to which an atom becomes electrically neutral on the whole. The arrangement of electrons inside the continuous positive charge is similar to that of the seeds in a watermelon or the plumps in a pudding.

Gold foil is a very thin sheet of gold metal. Such a gold foil was used in Rutherford’s alpha particles scattering experiment which led to the discovery of the nucleus.

Alpha Particle Scattering Experiment

Thomson's Model of an Atom
Alpha particle  experiment


When fast-moving alpha particles are allowed to strike a very thin gold foil in a vacuum, it is found that:

  • Most of the alpha particles pass straight through the gold foil without any deflection from their original path; it shows that there is a lot of empty space in the atom.
  • A few alpha particles are deflected through small angles and a few are deflected through large angles to show that there is a center of positive charge in the atom which repels the positively charged alpha particles and deflects them their original path. This center of positive charge in the atom is known as the nucleus.
  • The observation that very few alpha particles completely rebound on hitting the gold foil and turn back on their path shows that that the nucleus is very dense and hard which doesn’t allow the alpha particles to pass through it.


Failure of J.J Thomson’s model of an atom 

  1. He could not explain the origin of several spectral in the case of hydrogen and other atoms.
  2. He failed to explain the large-angle scattering of alpha particles in Rutherford’s experiment.


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