The idea
The model of the atom changed five times in a century, and every change was forced by an experiment that the old model could not explain. This is the best story in chemistry for showing how science actually works.
timeline title How the model of the atom changed 400 BCE : Democritus : matter is made of indivisible pieces 1803 : Dalton : solid spheres, one kind per element 1897 : Thomson : plum pudding, negative electrons in positive dough 1911 : Rutherford : dense positive nucleus, mostly empty space 1913 : Bohr : electrons confined to specific shells 1926 : Quantum model : electrons as probability clouds
The experiment that broke each model
| Model | Killed by | What was seen |
|---|---|---|
| Dalton’s solid sphere | Thomson’s cathode ray tube | Something smaller than an atom, and negative |
| Thomson’s plum pudding | Rutherford’s gold foil | A few alpha particles bounced almost straight back |
| Rutherford’s nucleus | Atomic emission spectra | Elements emit only specific colours, not a smear |
Rutherford's own words, roughly
He described the gold foil result as being about as believable as firing a naval shell at tissue paper and having it come back at you. If the atom were uniform dough, that could not happen. Something tiny, dense, and positive had to be in there.
The lesson to take
A model is not “the truth”. It is the simplest thing that explains every observation so far. When an observation does not fit, the model changes — not the observation.
Curriculum connection
C2.2
research the role of experimental evidence in the development of various atomic models, and compare and contrast different models of the atom
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