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16th Dec 2019 @ 3 min read
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.
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.
Thomson's model was a milestone in the atomic world. In spite of its advantages, it has many disadvantages.
His atomic model did gain popularity until his own student Rutherford in 1911 found the presence of the nucleus through his gold foil experiment.
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