Plum Pudding Atomic Model by J. J. Thomson

The plum pudding atomic model or atomic theory is one of the earlier atomic theories. The model was proposed by J. J. Thomson, who is also known for the discovery of the electron. From his cathode-ray tube experiments, he realized that atoms consisted of negatively particles (electrons), which he called corpuscles. He imagined an atom as negatively charged particles floating in the positively charged soup and put forward his theory in 1904.

Postulates of Thomson's atomic model

1. According to Thomson, the atom was spherical in shape.
2. The atom consisted of the negatively charged particles.
3. The negatively charged particles or electrons were floating in a positively charged soup (or ocean).
4. An atom was electrically neutral. The positive and negative charge in the atoms were equal.
5. The positive charge in an atom was uniformly distributed.

Explanation

Thomson studied magnetic and electric properties of cathode rays. He described that cathode rays were negatively charged corpuscles, which were later renamed as electrons, and were constituents of atoms. He also calculated the mass-to-charge ratio of these particles. Using this ratio, Thomson concluded the size of these particles was even smaller than of a hydrogen ion.

At that time, it was known the atoms were electrically neutral. To explain this, he introduced the presence of the positive charge in an atom such that the positive charge counterbalances the negative charge. Therefore, the net charge on an atom remained zero. In those days, the knowledge of the nucleus not revealed. Thus, Thomson conceptualized the atom as:

…a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification…

As per the model, the negatively charged electrons experience the net electrostatic force by the positively charged sphere towards the centre of the atom. There is also a repulsive force between electrons.

The electrons in the cloud of uniform positive charge is similar to the plums in a pudding, a popular British dessert. Hence, the model got its name the plum pudding model.

Limitations/drawbacks

Thomson's model was a milestone in the atomic world. In spite of its advantages, it has many disadvantages.

1. It did not mention the presence of the nucleus in an atom.
2. The model says the positive charge is uniformly distributed throughout the atom. This is highly untrue. We know the positive charge is centred in the atoms.
3. It failed to explain Rutherford's gold scattering experiment. The model was abandoned as it was unable to explain the deflection of alpha particles when projected on gold foils.
4. The model was rejected because it was ineffective to justify light spectrum. According to the model, the hydrogen atom can emit light only at one frequency. However, the hydrogen atom can emit light at different frequencies.

His atomic model did gain popularity until his own student Rutherford in 1911 found the presence of the nucleus through his gold foil experiment.

Associated articles

• J. J. Thomson's Cathode Ray Tube Experiments