Alpha Ray Scattering Experiment

Many physicists tried explaining the structure of an atom. J.J. Thomson first suggested the model for the structure of an atom. This model considered the structure of an atom similar to a plum pudding. But his model failed to explain certain important elements in an atom. Ernest Rutherford was a renowned British physicist. He proposed a model which contradicted the Thomson model. Rutherford performed the classic alpha ray scattering experiment to understand the structure of an atom. Following sections on alpha ray scattering experiment notes will give you a thorough understanding of the topic.

The distinction between plum pudding and the planetary model of an atom

Rutherford’s model of an atom

Rutherford gave the following conclusions on the structure of an atom. This was after he conducted his classic experiment.

According to Ernest Rutherford’s model of the atom:

1. It has a small, concentrated, positively charged nucleus

2. All mass is concentrated

3. Electrons are lightweight and negatively charged

4. Electrons revolve around it at very high speeds

5. The path of electrons is referred to as an orbit

6. Also called the planetary model

7. Atom has space mostly

8. The path of electrons can be predicted

9. Electrons and nucleus are clenched together by a strong force

10. The atom as a whole is neutral electrically

11.  It is also called the nuclear model of an atom

Ernest Rutherford’s Alpha ray scattering experiment

A thin gold sheet of 100 nm thickness was used. It was selected because a very thin layer was required. A high-energy ray of 𝛂 particles was bombarded on it. 𝛂 particles are helium ions that are doubly charged. The mass of these particles is around 4μ. They have a considerable amount of energy. It was provided by a radioactive source. A zinc sulphide screen was used around the foil. It was done to observe the deflection of 𝛂 particles. Rutherford wanted to know the arrangement of electrons in an atom. He studied the path of 𝛂 particles after their interaction with gold foil. 

Observations from the experiment

Important observations from Rutherford’s alpha ray scattering experiment were:

1. Most of the 𝛂 particles passed by the gold sheet without getting deflected.

Inference:

• There is a lot of space in an empty atom.

2. Few 𝛂 particles get deflected by the gold sheet at very small angles.

Inference:

• The positive charge in an atom is condensed.

• It is not uniformly distributed in the atom.

3. Very few particles got deviated at large angles or were rebounded.

Inference:

• Positive charge occupies a very small volume in an atom.

Limitations of the experiment

• Rutherford’s model explained that electrons revolve around the nucleus in fixed orbits. Accelerated charged particles give away electromagnetic radiation, according to Maxwell. Therefore, electrons in Rutherford’s model of an atom will radiate electromagnetic radiation. Electrons will provide the energy to the radiation. This will cost orbit shrinkage. Hence, the electrons will collapse in the core. It will take approximately 10-8 seconds. 

inference:

It could not account for an atom’s stability.

• Nothing specific was observed about the arrangement of electrons in an atom.

Inference:

Rutherford’s theory was incomplete.

Important formulas for alpha ray scattering experiment

1. The number of scattered alpha particles is given by

N ∝ 1 sin4(𝜃/2)

2. N/t = constant, where

N = number of alpha particles scattered in a direction

t = thickness of foil

3. The closest distance to the nucleus till which alpha particles could reach was given by r0. This is termed as the distance of the closest approach. Also, the early kinetic energy is transformed into potential energy. The relation between the closest distance of approach and kinetic energy is given by,

r0 = 4kZe2/mv2

Conclusion

In Rutherford’s alpha scattering experiment, he discovered a very small dense region composed of positively charged particles and hence named this part nucleus. After scattering the alpha particles over the gold sheet, it was observed that most of the particles went straight through the gold foil and there was no deflection. From this, it can be concluded that most of the spaces in atoms are empty.Maxwell electrons release electromagnetic radiation. But if we look into Rutherford’s atomic model, if the electrons release electromagnetic radiations then they will collapse and gradually shrink. Therefore the electrons will lose energy constantly due to accelerated charged electrons. This one is a major drawback of Rutherford’s atomic model.