The now obsolete plum pudding model was the first scientific model of the atom with internal structure. It was first proposed by J. J. Thomson in 1904 following his discovery of the electron in 1897 and subsequently rendered obsolete by Ernest Rutherford's discovery of the atomic nucleus in 1911. The model tried to account for two properties of atoms then known: that there are electrons and that atoms have no net electric charge. Logically there had to be a commensurate quantity of positive charge to balance out the negative charge of the electrons, but having no clue as to the source of this positive charge, Thomson tentatively proposed it was everywhere in the atom, the atom being in the shape of a sphere for the sake of mathematical simplicity. Following from this, Thomson imagined that the balance of electrostatic forces in the atom would distribute the electrons in a more or less even manner throughout this hypothetical sphere.[1]
Thomson attempted without success to develop a complete model that could predict any other known properties of the atom such as emission spectra or valencies. Based on experimental studies of alpha particle scattering, Ernest Rutherford developed an alternative model for the atom featuring a compact nuclear center. This model was taken up by Niels Bohr as the basis of the first quantum atom model.
Thomson's model is popularly referred to as the "plum pudding model" with the notion that the electrons are distributed with similar density as raisins in a plum pudding. Neither Thomson nor his colleagues ever used this analogy.[2] It seems to have been conceived by popular science writers to make the model accessible to the layman. The analogy is perhaps misleading because Thomson likened the sphere to a liquid rather than a solid since he thought the electrons moved around in it.[3]
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