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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.

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The streams of positively charged particles are called anode rays or positive rays or canal rays.
In 1886, Goldstein performed a discharge tube experiment, using perforated cathode. He observed that in addition to cathode rays, a new kind of rays were also found. These rays passed through the hole of the perforated cathode but travelled in a direction opposite to that of cathode rays. These rays were found to consist of positively charged particles and were called anode rays or positive rays or canal rays.

These rays are believed to be produced as a result of the knock out of the electrons from the gaseous atoms by the bombardment of high speed electrons of the cathode rays on them. Thus(anode rays are not emitted from the anode but are produced in the space between the anode and the cathode. In other words anode rays (or positive rays) are atomic or molecular residues from which some electrons have been removed. The removed electrons constitute the cathode rays and positive residues form the positive or canal rays.

Properties of anode rays:
(i) Anode rays travel in a straight line.
(ii) These rays are deflected by electric and magnetic fields in a way that shows that these rays are positively charged particles. For example, these rays are attracted towards the negative plate in the electric field which means rays are positively charged particles.
(iii) These rays consist of material particles and rotate the paddle wheel placed in their path.
(iv) Anode rays produce heating effect when struck against a metal foil.
(v) Hie ratio of charge to mass (^e/_m) for positive rays is considerably smaller than for electrons and is not constant but depends on the nature of the gas taken in the discharge tube. In other words, the value of ^e/_m for a positive ion depends upon the charge of the ion and its mass.

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Production of cathode rays: When a current of high voltage (10000 volts) is passed through a gas or air kept at a very low pressure (0.01 -0.03 mm) contained in a discharge tube, blue rays are seen emerging from the cathode which produce a fluorescence on the walls of the discharge tube. These rays are called cathode rays.

Properties of cathode rays:
1. Cathode rays travel in a straight line.
2. Cathode rays rotate the light paddle wheel when placed in their path. This shows that these rays consist of material particles having both mass and velocity.
3. Cathode rays produce green fluorescence upon striking glass or certain other materials.
4. When cathode rays are allowed to strike on a piece of metal foil, it becomes hot which shows that these rays produce heating effect.
5. These rays ionise the gas through which they pass.
6. These rays produce X-rays when they strike against hard metals such as tungsten.
7. When an electric or magnetic field is applied on the cathode rays, they are deflected from their straight path towards the positive plate of the electric field. This shows that cathode rays are negatively charged particles. These particles were named electrons.

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According to Rutherford’s model, electrons are revolving around the nucleus in circular orbits. The centrifugal force (due to the circular motion of electrons) acting outwards balances the electrostatic force of attraction (between the positively charged nucleus and negatively charged electrons) acting inwards. This prevents the electrons to fall into the nucleus.

It was shown by Clark Maxwell that a charged body moving under the influence of attractive force loses energy constantly. Thus, unlike a planet, electron is a charged particle and it should continuously emit radiation and lose energy. As a result of this, a moving electron will come closer and closer to the nucleus and after passing through a spiral path, it should ultimately fall into the nucleus. It has been calculated that it should take only 10^-8 sec for the electron to fall into the nucleus. But it is known that electrons keep on moving outside the nucleus. Hence there must be something wrong with Rutherford's atomic model itself.

This model also fails to explain hydrogen spectrum.



Fig.  The orbit becoming smaller and smaller as electron comes closer to the nucleus through spiral path

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Discharge tube experiments have revealed that electrons constituting the cathode rays are identical irrespective of material of cathode or gas used in the discharge tube. All these electrons are found to have same value of  ^e/_m . Thus cathode rays consist of fundamental common particles known as electrons.

Moreover, electrons can be also emitted:

(i) by heating certain metal filaments to high temperature,
(ii) by the radioactive substances in the form of β-rays and
(iii) by exposing certain metal surfaces especially the alkali metals to certain suitable high frequency radiations such as ultraviolet or X-rays.

From the above discussion, it is clear that electrons can be emitted from all kinds of matter and value of ^e/_m is found to be the same regardless of the source or method by which electrons are obtained. Therefore, it can be concluded that electrons are universal (or common) constituents of all matter.