Rutherford Scattering

Rutherford Scattering Experiment

The Rutherford scattering experiment was a key investigation into the structure of the atom. In this experiment, a beam of alpha particles (which are positively charged helium nuclei) was directed at a very thin sheet of gold foil. Alpha particles were used because they are positively charged and relatively heavy, making them suitable probes to investigate atomic structure.

Observations from the experiment showed:

• Most alpha particles passed straight through the gold foil without any deflection.
• Some alpha particles were deflected at small angles.
• A very small number of alpha particles were deflected at very large angles, some even bouncing back toward the source.

These results were surprising at the time because the prevailing atomic model (the plum pudding model) suggested that positive charge was spread out evenly through the atom, so large deflections were not expected.

Conclusions about Atomic Structure

From the scattering results, Rutherford and his team concluded the following about the atom:

• Atoms are mostly empty space: Since most alpha particles passed through the foil without deflection, the majority of the atom must be empty space.
• There is a small, dense, positively charged nucleus at the centre: The few alpha particles deflected at large angles must have come close to a very small, concentrated positive charge inside the atom. This is the nucleus.
• Electrons orbit the nucleus: The electrons, which are negatively charged, must be spread around the nucleus, orbiting it at some distance.

This new model replaced the plum pudding model and became known as the nuclear model of the atom.

For instance, if an alpha particle is fired at the gold foil and is deflected by a certain angle, this indicates it came close to the nucleus, which exerts a strong repulsive force due to its positive charge.

For example, if 1 in 8000 alpha particles is deflected at a large angle, this shows the nucleus is very small compared to the overall size of the atom.

Significance of Rutherford Scattering

The Rutherford scattering experiment was very important because it:

• Disproved the plum pudding model: The plum pudding model suggested positive charge was spread out evenly, but the experiment showed a concentrated positive nucleus.
• Led to the nuclear model of the atom: This model shows a tiny nucleus with electrons orbiting around it, explaining the scattering results.
• Laid the foundation for modern atomic theory: Understanding the nuclear structure of atoms was essential for later discoveries, including the development of quantum mechanics and nuclear physics.

Learning Example

If an alpha particle is fired at gold foil and most pass straight through, what does this tell us about the size of the nucleus compared to the atom?

Since most alpha particles pass through without deflection, the nucleus must be very small compared to the overall size of the atom. The atom is mostly empty space, with the tiny nucleus at the centre.