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The 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, but it was 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 an equal amount of positive charge to balance out the negative charge of the electrons. As he had no idea of the source of this positive charge, Thomson tentatively proposed that 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 more or less evenly throughout this hypothetical sphere. Thomson attempted without success to develop a complete model that could predict other known properties of the atom, such as emission spectra and 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 uniformly, like raisins in a plum pudding. Neither Thomson nor his colleagues ever used this analogy. 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.