|
|
The 'Plum Pudding' Model of the Atom and Rutherford's Experiment
|
|
|
|
Early Views of the Atom |
|
1. |
Around 400 BC a Greek scientist called Democritus said that matter was made up of small particles he named 'Atoma' (meaning indivisible). |
|
2. |
In 1804 John Dalton stated that matter consisted of tiny solid balls he called 'Atoms'. |
|
|
The 'Plum Pudding' Model of the Atom |
|
|
Around 1897 a scientist called J.J. Thompson (pictured left) showed that atoms contained small particles with a negative charge. |
|
He named the particles electrons. |
|
|
Thompson knew that atoms had no charge, i.e. were neutral. |
|
|
This meant a positive charge in the atom must be present to balance the negative charge of the electrons. |
|
|
Thompson proposed a model for the structure of an atom from his experiments. |
|
|
He said the negatively charged electrons were embedded in a sphere of positive charge so the charges were balanced. |
|
|
This became known as the 'plum pudding' model of the atom and is shown in the diagram below. |
|
|
|
|
|
Rutherford's Alpha Particle Experiment |
|
The 'plum pudding' model for an atom's structure was accepted until the experiments of Ernest Rutherford, a scientist from New Zealand, in 1910. |
|
Rutherford (pictured right) fired positively charged particles called alpha particles at a thin sheet of gold foil. |
|
|
The alpha particles were emitted from a sample of Uranium. |
|
|
Rutherford expected all the alpha particles to go through the foil, as he believed Thompson's 'plum pudding' atomic structure. |
|
|
His experiment is animated below. |
|
|
|
|
Rutherford found that: |
|
|
|
1. |
A small number of alpha particles were deflected at various angles to their initial direction. |
|
|
2. |
A small number of alpha particles rebounded off the foil back towards the sample of Uranium. |
|
|
The experimental results are shown, at an atomic scale, on the diagram to the right. |
||
|
Rutherford concluded that: |
|
|
1. |
An atom's mass must be concentrated in a small positively charged nucleus as only a very small number of alpha particles either deflected or rebounded off the foil. |
|
2. |
Most of the atom must be empty space. This space must contain the electrons. |
|
|
|
VIEW CONTENTS |
