Rutherford Atomic Model Experiment Observations and conclusions - Gold foil experiment Key feature

The so-called Rutherford’s experiment, is a case in point of the wonder around the history at the educational level of one of the decisive events of modern science: The pertinent question as to how the constituent subatomic particles of atoms are placed?

Rutherford gave the answer when he discovered that the atom has a nucleus. In 1906, after having studied a number of properties of natural radioactivity, Rutherford directed his research programme to study the interaction of alpha particles to better understand the Plum Pudding model of an atom. Now it begs the question how the interaction of alpha particles with an atom would have occurred. Before understanding the whole experiment, let us remember the statement made by Rutherford which goes as follows “It is as if one had fired a large naval shell at a piece of tissue paper and it had bounced back”

Ernest Rutherford conducted a gold foil experiment and based on the observations of this experiment, he proposed the atomic structure and gave the Rutherford Atomic Model.

Table of contents:

• Gold foil experiment
• Observations and conclusions
• Key feature of Rutherford atomic model
• Drawbacks of Rutherford’s Model
• Practise Problems
• Frequently Asked Questions-FAQs

Gold foil experiment

• A stream of high energy α–particles from a radioactive source was bombarded on a thin foil of gold metal.
• The thin gold foil was surrounded by a circular observer screen coated with fluorescent zinc sulfide.
• Whenever an α–particle collided with the screen, a tiny flash of light was produced at that point on the screen.

Observations and conclusions

• Most of the -particles passed through the gold foil without any deflection. This proved that most of the space inside the atom is empty.
• Few alpha particles were deflected by small angles and a very few alpha particles (1 out of 20,000) bounced back completely (180°). So, positive charge is concentrated in a very small region and not uniformly distributed in the whole atom and there is a small positively charged core at the center.
• Atom consists of a small positively charged core at the center that almost carries the entire mass of the atom, known as the nucleus of an atom.

Key feature of the Rutherford atomic model

• Nucleus is surrounded by electrons revolving in circular paths known as orbits.
• Electrons and the nucleus are held together by electrostatic forces of attraction in an atom.
• The volume of the nucleus is negligible when compared to the total volume of the atom.
• The radius of the atom is about 10^-10m, whereas the radius of the nucleus is 10^-15m.

Drawbacks of Rutherford’s Model

• The Rutherford atomic model was not able to explain the stability of an atom. According to the classical theory of electromagnetism proposed by Maxwell theory, every accelerated charged particle must emit radiation in the form of electromagnetic waves and lose energy. Since the energy of electrons keeps on decreasing, the radius of the circular orbits should also decrease and ultimately the electron should fall into the nucleus.        

Practice problems

Q 1. Approximate percentage of alpha particles which was bounced back with almost 180° is                                                                  

1. 5 %
2. 0.5 %
3. 0.05 %
4. 0.005 %

Solution: D

Few alpha particles were deflected by small angles and a very few alpha particles (1 out of 20,000) bounced back completely (180°).

% of alpha particles bounced back with 180° =  = 0.005 %

Q2. Atoms have a nucleus. It was first suggested by  

1.  Rutherford
2. Thomson  
3. Lenard
4. Dalton

Answer: A

Solution:   Based on the observations of the alpha scattering experiment, Rutherford concluded that atoms consist of a small positively charged core at the center known as the nucleus of an atom.

Q3. According to Rutherford's atomic model, which subatomic particle is present in the nucleus of an atom?

1. Electron
2. Proton
3. Neutron
4. None of these

Answer: B

Solution: Due to observed deflection in alpha particles from their initial path,  Rutherford concluded that the atom consists of a small positively charged core at the center that almost carries the entire mass of the atom, known as the nucleus of an atom.

Q4. What is the charge to mass ratio of alpha particle and proton

1. 1:1
2. 1:2
3. 2:1
4. 4:1

Answer: B

Solution: Let charge of proton = 

and mass of proton = 

So, Charge of helium = 

and mass of helium = 

Frequently Asked Questions-FAQs

Question 1. What are radioactive nuclei?

Answer: The majority of atomic nuclei are stable in nature, they do not decay over time but unstable nuclei will attempt to reach stability by ejecting nucleons (protons or neutrons), as well as other particles, or by releasing energy in other forms.

Question 2. Name the radiations which are generally emitted by radioactive nuclei.

Answer: The radiation that emits spontaneously from the nuclei of unstable nuclei is generally alpha, beta, or gamma radiation.

Question 3. what was the source of  in  scattering experiment.

Answer: Rutherford used radium as the source of the alpha-particle beam in alpha scattering experiment to bombard a thin foil of gold.

Question 4. Why fluorescent ZnS screens are used in alpha scattering experiments ?

Answer: Whenever α–particles strike the screen, a small flash of light is produced at that point.

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