The History of the Atom: Dalton, Thomson, Rutherford & Bohr (MYP Chemistry)
The atom is not something anyone has ever seen with the naked eye, so our picture of it was built from evidence, one experiment at a time. Each new model kept what the evidence supported and replaced what it contradicted. Understanding that story — model, evidence, revised model — is what MYP asks for when it says "models" and "evidence".
Try it yourself: travel through the history of atomic models as a platformer, meeting each scientist and their evidence.
Early ideas and Dalton
Long before modern experiments, thinkers in several cultures — including the ancient Indian philosopher Kanad — proposed that matter is made of tiny indivisible particles. In the early 1800s John Dalton turned this into a scientific model: elements are made of identical atoms, atoms of different elements differ, and atoms are solid, indivisible spheres that rearrange in chemical reactions.
Thomson: the electron and the plum pudding
In 1897 J.J. Thomson showed that atoms contain much smaller negatively charged particles, electrons. Since atoms are neutral overall, he pictured a positive "pudding" with electrons scattered through it like plums. Atoms were no longer indivisible — the first big revision of Dalton's model.
Rutherford: the nucleus
Rutherford's team fired positively charged alpha particles at thin gold foil. Most passed straight through, a few were deflected, and a very few bounced almost straight back. That could only happen if the positive charge and most of the mass were packed into a tiny, dense nucleus, with the rest of the atom mostly empty space. The plum pudding could not explain it.
Bohr: electrons in energy levels
A nucleus with orbiting electrons raised a problem: the electrons should lose energy and spiral in. Niels Bohr proposed that electrons can only occupy certain fixed energy levels (shells) around the nucleus and move between them by absorbing or emitting specific amounts of energy. This explained why elements give off light of only certain colours, and it is the shell model still used to draw electron arrangements today.
How to answer "explain how the model changed"
A strong answer names the model, states the new evidence, and says what that evidence showed the old model could not explain. For Rutherford: "gold foil results — a few alpha particles deflected — showed a small dense positive nucleus, which the plum pudding model could not account for."
Frequently asked questions
That an atom has a small, dense, positively charged nucleus with mostly empty space around it, because most alpha particles passed through and a few were deflected strongly.
It could not explain why some alpha particles were deflected through large angles; that needed a concentrated positive nucleus.
The idea that electrons occupy fixed energy levels (shells) and jump between them by absorbing or emitting specific amounts of energy.