Development of the model of the atom
Emmanuel Opoku
Teacher
Contents
Changing Scientific Models
Scientific ideas change over time as new evidence is discovered.
Each new experiment gives scientists a better understanding of the atom’s structure.
The atomic model evolved through several stages — from indivisible spheres, to the plum pudding model, to the nuclear model, and finally the modern atomic model.
Model | Scientist | Year | Description |
Solid sphere
| Dalton | 1803 | Atoms were thought to be tiny, solid spheres that could not be divided or broken down. |
Plum pudding
| Thomson | 1904 | The atom was thought to be a ball of positive charge with negative electrons embedded within it — like plums in a pudding. This model suggested that positive and negative charges were spread evenly throughout the atom. |
Nuclear
| Rutherford | 1911 | The atom has a small, dense, positively charged nucleus at its centre. Electrons move around the nucleus in the surrounding empty space. |
Planetary
| Bohr | 1913 | Electrons orbit the nucleus at specific distances or energy levels. |
The Alpha Particle Scattering Experiment
Evidence to support the nuclear model was gathered in 1909 by Ernest Rutherford, assisted by Hans Geiger and Ernest Marsden, with the alpha particle scattering experiment.
They fired alpha particles (positively charged) at thin gold foil and recorded how they were deflected.
If the plum pudding model was correct, all the alpha particles would pass straight through as the positive charge of the atom would be too spread out to repel them.
However, the evidence below was collected:
Observation (Evidence) | What is showed |
Most alpha particles passed straight through the foil. | Atoms are mostly empty space. |
Some alpha particles were deflected slightly. | The positive charge is concentrated in a small central area. |
A few particles were deflected backwards. | Most of the mass is concentrated in a tiny nucleus. |
Discovery of Protons and Neutrons
Later experiments showed that the positive charge in the nucleus was carried by smaller particles called protons.
Around 1932, James Chadwick provided experimental evidence for neutrons, neutral particles within the nucleus.
This discovery completed the modern atomic model — with protons and neutrons in the nucleus and electrons in shells around it.
Subatomic particle | Year discovered |
Electron | 1897 |
Proton | 1919 |
Neutron | 1932 |
Question 1:
Describe how evidence from the alpha particle scattering experiment led to a change in the atomic model.
Answer:
Most alpha particles passed through the foil, showing atoms are mostly empty space.
Some were deflected, suggesting a small, dense, positively charged nucleus.
This evidence replaced the plum pudding model with the nuclear model.
Question 2:
State two differences between the plum pudding model and the nuclear model of the atom.
Answer:
The plum pudding model described the atom as a ball of positive charge with electrons spread through it, while the nuclear model had a central positive nucleus with electrons orbiting around it.
The plum pudding model suggested the atom was solid, but the nuclear model showed that it is mostly empty space.
Summary
Early models viewed the atom as indivisible or a solid sphere.
Thomson’s plum pudding model introduced electrons in a positive “pudding”.
Rutherford’s scattering experiment proved the atom has a dense, charged nucleus.
Bohr introduced electron orbits (shells), explaining atomic behaviour.
Chadwick discovered neutrons, completing the modern nuclear model.
Each stage of discovery shows how new evidence leads to changes in scientific models over time.
No answer provided.