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{{use British English|date=October 2024}}
[[File:Thomson atom seven electrons.svg|right|thumb|An atom with seven electrons arranged in a pentagonal dipyramid, as imagined by Thomson in 1905]]
The '''plum pudding model''' is an obsolete scientific model of the [[atom]]. It was first proposed by [[J. J. Thomson]] in 1904 following his discovery of the [[electron]] in 1897, and was 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 Thomson had no idea as to the source of this positive charge, he tentatively proposed that it was everywhere in the atom, and that the atom was spherical. This was the mathematically simplest hypothesis to fit the available evidence, or lack thereof. In such a sphere, the negatively charged electrons would distribute themselves in a more or less even manner throughout the volume, simultaneously repelling each other while being attracted to the positive sphere's center.<ref>{{harvnb|Thomson|1907|p=103}} "In default of exact knowledge of the nature of the way in which positive electricity occurs in the atom, we shall consider a case in which the positive electricity is distributed in the way most amenable to mathematical calculation, i.e., when it occurs as a sphere of uniform density, throughout which the corpuscles are distributed."</ref>
Despite Thomson's efforts, his model couldn't account for [[emission spectra]] and [[Valence (chemistry)|valencies]]. Based on experimental studies of alpha particle scattering (in [[Rutherford scattering experiments|the gold foil experiment]]), [[Ernest Rutherford]] developed an alternative [[Rutherford model|model for the atom]] featuring a compact nucleus where the positive charge is concentrated.
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