Development of the Atomic Model Part 1 (GCSE Physics)

Development of the Atomic Model Part 1

Discovery of the Electron

The atomic model has developed greatly over time. Atoms were thought to be the smallest particle before the electron was discovered. Scientists thought that it was a very small dense sphere, which could not be split into pieces.

Plum Pudding Model

• The discovery of the electron changed scientific thoughts. Once scientists discovered the electron, they changed the model of the atom. Originally, they thought that an atom was just one single sphere.
• Electrons were thought to be buried in the atom. Now that scientists knew that electrons existed, they thought that they must be buried inside the atom. They thought that the atom was positive, with negative electrons embedded inside (see figure 1).

The Nuclear Model

• Scientists investigated the atom further. A scientist called Rutherford investigated the atom further with his two assistants. He decided to perform an experiment, which was called the alpha particle scattering experiment.
• They aimed an alpha particle beam at a gold sheet. The scientists used a gold foil sheet, at which they fired positive alpha particles in a beam from a source. When performing the experiment, they found that some particles went through the sheet. Other particles seemed to ‘bounce off’ the sheet and come straight back to the alpha particle source, like a boomerang.
• The plum pudding model couldn’t explain the particles’ behaviour. The model of the atom at the time (the plum pudding model) couldn’t explain why some of the alpha particles came straight back to where they came from.

1. The mass of the atom is must be concentrated at the centre of the nucleus, since only a few of the alpha particles were deflected back.
2. The nucleus of the atom must be charged (positively), since it was repelling some of the positive alpha particles away.
3. The nucleus of the atom must be small, since only a few of the alpha particles were repelled (not all of them).
4. Most of the atom must be empty space, since most of thealpha particles passed through without being repelled by the nucleus.

Differences between the 2 Models