Rutherford’s Theory & Atomic Model with its Observation

Key Concepts

• Atomic model
• Rutherford atomic model
• Alpha ray scattering

Introduction: 

Atomic models: 

Observations and experiments mentioned by some scientists suggested that Dalton’s indivisible atom is composed of sub-atomic particles carrying positive charge and negative charge. The major problems recognized by the scientists after the discovery of sub-atomic particles and the atomic models were:  

1. Stability of atom 

2. Comparing the elements behavior with both the physical and chemical properties 

3. Formation of molecules combining with identical atoms 

Explanation: 

Atomic models were proposed to explain about the charged particles in an atom.  

Two of these models, one proposed by J.J. Thomson and the other proposed by Ernest Rutherford.  

Bohr’s model explains how electrons revolve around the nucleus. 

 J.J Thomson’s atomic model failed to provide experimental results. This led to the discovery of another model of atom that is Rutherford’s atomic model. 

Rutherford model of atom:  

Rutherford (1871-1937) was born at Nelson, New Zealand. He studied at the University of New Zealand and conducted experiments and research works with J.J Thomson at cavendish laboratory. He won the noble prize in the year 1908 in the field of atomic physics.  

“All science is either physics or stamp collecting.” —- Ernest Rutherford. 

Rutherford designed an experiment to probe inside the atoms. Rutherford and his students bombarded gold foil with alpha-particles. He proposed atomic model based on his famous alpha-ray scattering experiment.  

The experiment is explained as below: 

A beam of α- particles taken from a high radioactive source is passed through the thin gold foil of metal enclosed with fluorescent ZnS screen around it. The α-particle became visible whenever it struck the screen and made small angles as they passed through the foil.

Observations and conclusions: 

1. Most of the α-particles passes through the foil without any deflections this means that most of the atom is empty. 

2. A small fraction of the α-particle deflected from the original path. This indicated that the positive charge is at the centre of the atom and due to repulsive forces deflections take places. 

3. A very few α-particles (1 in 20,000) bounced back which indicated that the whole mass is concentrated at the centre of atom.  

4. The radius of the atom is very small and in the order of 10^-10 m, whereas for nucleus it is  
   10^-15m. 

5. The electrons revolve around the nucleus like planets revolve around the sun, hence, this model is called planetary model or nuclear model.

On the basis of the above observations and conclusions, Rutherford proposed the atomic structure of elements or nuclear model of atom. According to this model: 

1. The positive charge and most of the mass of atom was more densely concentrated in extremely small region. 

2. The very small portion or region or the heaviest part was named  “nucleus” by Rutherford. 

3. The nucleus is surrounded by electrons that revolve around the nucleus in a circular path called orbits or shells. 

4. Electrons are negatively charged. Nucleus and mass of positively charged particles are held together by electrostatic force of attractions. 

5. One particle, later called proton, had a positive electric charge, and the other particle, called neutron, had no electric charge 

Drawbacks of Rutherford atomic model of atom:  

1. This model failed or could not explain the stability of atom. 

2. It couldn’t explain the electronic structure of atom and energies of electrons. 

3. This model is against the law of thermodynamics. 

Summary:

1. The nucleus is surrounded by electrons that revolve around the nucleus in a circular path called orbits or shells.
2. Most of the atom is empty.
3. A small fraction of the a-particle was deflected from the original path. This indicates that the positive charge is at the centre of the atom.
4. Rutherford is famous for alpha-particle scattering experiment.
5. A beam of a- particles taken from a high radioactive source is passed through the thin gold foil of metal enclosed with fluorescent ZnS screen around it.
6. The a-particle became visible whenever it struck the screen and made small angles as they passed through the foil.
7. One particle, later called proton, had a positive electric charge, and the other particle, called neutron, had no electric charge.