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In 1803, English chemist John Dalton proposed that all matter consists of tiny, indestructible spheres called atoms.
For nearly a century, Dalton's sphere model explained chemical reactions perfectly โ until a brand-new experiment fired particles directly at gold foil and shattered everyone's expectations.
What happens when experimental data directly contradicts what every scientist believes to be true?
The Scientific Cycle
A scientific theory is a well-tested explanation of natural phenomena, while a scientific model provides a physical, conceptual, or mathematical representation to generate testable predictions.
When new instruments yield observations that contradict a model, scientists cannot ignore the data; they must either modify the model or replace it entirely.
๐Interactive diagram
How did this cycle completely reshape our view of the atom across four major discoveries?
Case Study: The Evolving Atom
In 1897, J.J. Thomson discovered the electron โ a subatomic particle with negative charge and negligible mass โ proving atoms were not indivisible.
Thomson proposed the plum pudding model, describing the atom as a cloud of positive charge with negative electrons embedded throughout like raisins in pudding.
In 1909, Ernest Rutherford tested Thomson's model by firing positively charged alpha particles at an ultra-thin sheet of gold foil.
While most alpha particles sailed straight through, about 1 in 8,000 deflected at huge angles or bounced straight back โ a physical impossibility under the plum pudding model.