92.

How is position of electron and proton in an atom established? Describe an experiment in this regard.
Or
Describe Rutherford's scattering experiment. How did it lead to Rutherford’s nuclear model of atom?
Or
Describe an experiment which provides evidence that most of the charge and mass of an atom is concentrated in its nucleus.

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The position of electrons and protons in an atom was established by an experiment performed by Rutherford in 1911. It is also called x-ray scattering experiment.



Fig. Rutherford’s α-particles scattering

Scattering experiment: Rutherford (1911) allowed a narrow beam of α-particles (He²⁺ions) to fall on a very thin gold foil (thickness nearly 0 . 0004 cm) and determined the subsequent path of these particles with the help of spherical zinc sulphide coated screen.

Observations. Rutherford observed that:
(i) Most of the α-particles (nearly 99%) passed through the gold foil with little or no disturbance.
(ii) A few α-particles were deflected from their paths through moderate angles.
(iii) Very few α-particles (approximately 1 out of 20000) were deflected back through an angle greater than 90°. Some actually deflected light back (by 180°).


Fig. Deflection of α particles. Nearer the particles come to the nucleus, larger is the angle of deflection.

On the basis of the above observations about the scattering of alpha particles, he proposed a model of the atom called Rutherford model of the atom.

Rutherford’s atomic model. According to this model:
1. Almost the entire mass and all of the positive charge of an atom is concentrated in a very small region at the centre of the atom known as a nucleus.

2. The size (diameter) of the nucleus (nearly 10^-13 cm) is very small as compared to the size of the atom (nearly 10^-8cm).

                    



3. Most of the space outside the nucleus is empty and is called extra nuclear part.

4. The magnitude of the positive charge on the nucleus is different for different atoms.

5. The electrons, equal in number to the net nuclear positive charge, revolve around the nucleus in a scattered manner with a fast speed.

6. The centrifugal force arising due to the fast speed of an electron balances the coulomb force of attraction of the nucleus and the electron remains stable in its path.

Rutherford model of an atom can be compared with the solar system in which nucleus represents the sun and revolving electrons represent the planets. Due to this comparison, revolving electrons are sometimes called planetary electrons.