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💡 A puzzle
Thomson had found negative electrons — but atoms are neutral overall. So where is the positive charge hiding?
What is Thomson’s model of the atom?
Thomson's Model
A sphere of positive charge with electrons spread throughout it, keeping the atom neutral. Like seeds in a watermelon (plum pudding)
- Atom is a sphere of positive charge .
- The electrons are spread throughout it, keeping the atom neutral.
- Called the plum pudding model (like seeds in a watermelon).
ⓘ Note
- Atoms do not actually show any colour.
- Colours in these diagrams are only for illustration.
👤 Meet a Scientist — J. J. Thomson
- Discovered the electron — the first subatomic particle found.
- Won the Nobel Prize in Physics (1906) for his study of conduction of electricity in gases.
- Headed the famous Cavendish Laboratory , Cambridge, and guided Ernest Rutherford.
⏸▶ Pause and Ponder
- 1. If you build a Thomson ‘atom’ from clay (positive) and beads (electrons): (i) if the positive clay is less than the total negative of the beads, the model becomes negatively charged (not neutral); (ii) if the clay itself carries some negative charge, the total negative rises, so the model is no longer neutral .
- 2. An orange or lemon can work like a rough model — soft pulp (positive matter) with seeds (electrons) inside. It matches the “charges mixed together” idea, but falls short because the seeds sit in a pattern and it cannot show that the atom is mostly empty space.
- 3. Because cathode rays behaved the same whatever gas or cathode metal was used, Thomson concluded electrons are present in all atoms.
Important Definitions
- Thomson's model — the atom is a sphere of positive charge with electrons embedded throughout it (the plum pudding model).
