The Geiger-Marsden-Rutherford experiment discovered that an atom contains a very small, dense, positively charged nucleus holding most of its mass, while most of the atom is empty space. It therefore disproved the plum pudding model.
The Reasoning and Mechanism
This content is part of E.1 Structure of the atom and is required at both SL and HL. Geiger and Marsden directed positively charged alpha particles at a very thin sheet of gold foil and detected their paths using flashes produced on a surrounding screen.
| Experimental observation | Inference about atomic structure |
|---|---|
| Most alpha particles passed straight through the foil. | Most of the atom is empty space. |
| Some alpha particles were deflected through small angles. | The atom's positive charge is concentrated rather than spread uniformly throughout the atom. |
| A very small number were deflected through large angles or travelled backward. | Most atomic mass and positive charge are concentrated in a tiny, dense nucleus. |
Alpha particles are positively charged. Their deflection resulted from electrostatic repulsion when they passed close to the positively charged nucleus. Large-angle scattering was rare because the nucleus occupies only a tiny fraction of the atom's volume.
These observations contradicted the plum pudding model, which described positive charge as spread throughout the atom with electrons embedded within it. Under that model, an alpha particle should experience only weak, distributed forces, so large-angle deflections could not be explained.
A common misconception is that the experiment discovered the neutron or determined the detailed arrangement of electrons. It did neither. It established the nuclear model of the atom; the neutron was discovered later.
Exam Technique
For an IB Explain question, link each observation to its inference: most particles passed through, so the atom is mostly empty space; a few were strongly deflected, so positive charge and most mass occupy a tiny nucleus. Do not merely state that Rutherford “discovered the nucleus” without using the scattering evidence.