All about Observations of Rutherford’s Alpha Scattering Experiment

In 1911, British physician Ernest Rutherford performed a gold foil experiment. He used this to understand the structure of an atom. Ernest Rutherford carried out the experiment along with Hans Geiger and Ernest Marstden. It helped accurately understand the composition of an atom. Ernest Rutherford also received the Nobel Prize for identifying alpha particles with Helium and specified that these atoms of Helium are doubly ionised. He tested Thomson’s hypothesis by devising his ‘gold foil’ experiment. Rutherford’s model proved to be an essential step toward the complete understanding of the atom. However, it did not completely address the nature of an electron and how it occupied the vast space around the nucleus. His (Rutherford’s) experiment proved to be the key to understanding the chemical properties of elements.

Rutherford’s alpha scattering experiment is also known as Rutherford’s nuclear model of the atom because it was the first atomic model to feature a nucleus at its core.

Arrangement of the experiment 

1. Rutherford took a very thin gold foil of 100 nm thickness.
2. He bombarded it with Alpha particles. 

• Alpha particles are He++ atom Or He nucleus.
• The charge on alpha particles is +2 units.
• The mass of an alpha particle is 4 units.
• The Alpha particle source is Radium (Ra), discovered by Madam Curie.
• Alpha particles are radioactive particles or rays emitted by a radioactive element.

3. He also placed a zinc sulphide (ZnS) Screen behind the gold foil, which helped describe the pathway of an alpha particle.

Observation 

1. Most of the alpha particles passed through gold foil undeflected.
2. A small fraction of alpha particles were deflected by small angles.
3. Almost one alpha particle among 20,000 alpha particles bounced back, i.e. deflected nearly 180 degrees.

Based on these observations, Rutherford concluded the model of an atom and made the following observations:

Result and conclusion

1. Most of the space in an atom is empty.

2. Atoms contain a very dense and heavy core called the nucleus.

3. All the positive charge in an atom exists in the nucleus.

4. The nucleus is surrounded by electrons that move around it at a very high speed on a circular path called the orbit. Thus, Rutherford defined the model of an atom as a miniature of the solar system.

5. The total negative charge balances the total positive charge.

6. The total number of protons present in the nucleus is equal to the total number of electrons present in the empty space of the atom.

7. The backward bounce of some alpha particles as projectiles sent on the atoms of a thin gold foil was interpreted as the ‘backscatter’ of a light nucleus on the heavy nucleus of a gold atom.