An early model picturing electrons embedded in a diffuse positively charged sphere.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
J.J. Thomson proposed around 1904 that electrons are embedded like negative raisins in a diffuse, positively charged sphere, so that the atom is neutral overall. The model explained why atoms are electrically neutral and why electrons can be knocked out of matter. It also suggested that the positive charge is spread through the whole atom. The model was disproved by Rutherford's gold foil experiment, which showed that the positive charge and most of the mass are concentrated in a tiny nucleus. It remains a useful historical step in the development of atomic theory.
Key ideas: Thomson (1904) proposed a positively charged sphere with electrons embedded like plums, overall neutral. It was built from cathode-ray results: electrons exist in all matter. Prediction: alpha particles shot at a thin foil should pass through with only small deflections.
Common mistakes and connections: Students think the model failed because it had no electrons; it failed because charge is not spread out, as shown by large-angle scattering. It still correctly said atoms contain electrons. It is the baseline against which Rutherford's result is judged and leads to the nuclear model.
Thomson pictured electrons scattered through a diffuse positive sphere; helium would have 2 electrons embedded in a uniform positive cloud, a model later replaced. The interactive lesson shows this as a live animation or chart.
The plum pudding model predicted only small deflections of alpha particles, so the large deflections that were observed could not be explained by it.
An early model picturing electrons embedded in a diffuse positively charged sphere.
Thomson's Plum Pudding Model is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Atomic Structure & Periodicity first: Electron, Proton, Neutron.
The next topics in Atomic Structure & Periodicity are Rutherford's Gold Foil Experiment and Nuclear Atom Model.
Yes. The interactive lesson includes a self-check question on Thomson's Plum Pudding Model, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Thomson's Plum Pudding Model is topic 16 of 115 in Atomic Structure & Periodicity.