Discovery Of Electrons

The fundamental particle carrying a negative charge was discovered in 1897, by the British physicist J.J. Thomson. The apparatus used for this type of experiment is called the discharge tube (Neon sign and T.V. tube are examples of discharge tube) which consists of a glass tube, fitted with two metal electrodes connected to a high voltage source and a vacuum pump.

When the tube is evacuated and a current of high potential is passed between the electrodes, streaks of bluish light extending from negative electrode (cathode) towards positive electrode (anode). The rays appear to travel in straight lines, from the cathode to anode, cause the wall at the opposite end of the tube glow where they strike. These rays were called cathode rays. Thomson showed that these rays were deflected towards the positive plate in electric and magnetic field. This shows that these rays consisted of negatively charged particles. The name electron was given to these units of negative charges. Electrons were obtained irrespective of the nature of cathode or the gas in the discharge tube. This proves that electrons are constituents of all matter.

Properties of Cathode Rays:

1.    They cast shadows of objects placed in their path towards the anode, proving that they travel in straight lines.

2.    They cause a light paddle wheel to rotate (revolve). Showing that they are material particles.

3.    These rays although invisible cause some material to glow or produce fluorescence.

4.    They are deflected towards the positive plate in an electric field, showing that they are negatively charged particles.

5.    The (charge/mass) e/m ratio of cathode particles is 1.7588x108 c/g (Coulomb per gram), i.e same for all electrons, regardless of any gas in the tube.

6.    They can produce mechanical pressure indicating they possess kinetic energy (K.E.).

3.2.1 Discovery of Protons:

Since£ atoms are electrically neutral and electrons carry negative charge, it follows that for each electron, there must be one equivalent positive charge to neutralize that electron. This particle is called a proton. It is one of the fundamental units of structure of all atoms. The simplest atom of hydrogen (H) is therefore made up of one electron and one proton.

 e.g                        H       ---à   H+      +       e-

                                                 (Proton)    (Electron)

 Protons were first observed in apparatus similar to cathode rays tube, with a perforated cathode by German Physicist Goldstein in 1886, their existence was verified and their properties were investigated in 1897 by J.J. Thomson. 

Properties of positive rays:

1.    These rays also travel in straight line from anode to cathode.

2.    These are deflected towards the negative plate when passed through an electric field, showing that these carry a positive charge.

3.    The charge i.e., e/m ratio of positive particle is much smaller mass

4.    Than that for electron and it varies with the nature of gas in the tube.

3.2.2 Discovery of Neutrons:

In 1932 the English Physicist James Chadwick discovered a third type of fundamental particle of atomic structure through artificial radioactivity, further it will be discussed in the definition of artificial radioactivity.

Properties of Electrons Protons and Neutrons:

Electron:     

Electron is negatively charged particle. The charge measured in coulombs. The electric charge is equal to 1.602 x 10-19 coulombs. The electron carries a negligible mass. Its mass is 1/18360 part of that of proton the actual mass of an electron is 9.109 x 10-31 Kg.

Proton:

Proton is positively charged particle. The actual charge on a proton is 1.602x10-19 coulombs. A proton is 1836 times heavier than electron. The actual mass of a proton is 1.672 x 10-27 Kg.

Neutron:

The neutron is neutral particle, i.e. it has no charge. Its mass is almost equal to that of proton. The actual mass of a neutron is 1.76 x 10-27 Kg. Properties of three sub-atomic particles of the atoms are summarized in table no. (3.1)

Table 3.1

 

PARTICLE

 

MASS

 

CHARGE

 

 

GRAMS

Kilograms

a.m. u.

COULOMBS

      e.s.u.

CHARGE UNIT

Electron

9.109390x10-28

9.109390x10-31

0.0005485

-1.602177x10-19

-4.8x10-10

- 1

Proton

1.672623x10-24

1.672623x10-27

1.007276

1.602177x10-19

4.8x10-10

+1

Neutron

1.764929x10-24

1.76492x10-27

1.008664

NONE (0)

NONE (0)

NONE(0)

3.2.3 Radioactivity:

The first conclusive evidence that atoms are complex rather than indivisible as stated in the atomic theory came with the discovery of radioactivity by Henry Becquerel, a French Physicist in 1896.

Definition of Radioactivity:

Radioactivity is the spontaneous disintegration of nucleus of an atom, in which invisible radiations are emitted from the nucleus of atoms. The substances which emit such kind of radiations are known as radio-active elements and the phenomenon is termed as Radioactivity.

Nature of Radioactivity: (Types of rays)

The British physicist Ernest Rutherford in 1902 determined the nature of radioactive rays by the following experiment and showed that, it is composed of three types of rays.

A sample of radioactive substance was placed in a lead block, between the two oppositely charged plates (electric field). The radiations were resolved into three components. One component was deflected towards the negative plate, proving that it carried a positive charge. These were named ∞-rays. The second component deflected towards the positive plate, showing that it carried a negative charge. These were named (3-rays. The third type carried no charge no mass and were not deflected in the electric field. These were, named y-rays.

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