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Tuesday, 29 September 2015

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 1

FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 1


Every matter is composed of extremely small particles which may or may not have free existence. These particles are known as atoms. The word atom is derived from a Greek word “Atoms” which mean “Indivisible ". Thus Greek philosophers considered that atom is indivisible smallest particle of any matter. Later on it was known through experiments that although atoms cannot be broken down in ordinary chemical reactions but they are composed of many sub-atomic particles like proton, electron, neutron etc. The physical and chemical properties of atom are determined by the arrangement of these sub-atomic particles, in it. Therefore these sub-atomic particles are known as fundamental particles of atom.

Discovery of sub-atomic particles and knowledge about the modern structure of atom is obtained from the following studies.

  1. X—rays
  2. Study of electron discharge through gases
  3. Radioactivity
  4. Spectroscopic studies

Nature of Charged Particles:

The particles having negative or positive charge are known as charged particles. Eg Electron, proton, etc
Some important features of charged particles are as follow:

  1. The movement of charged bodies in a conductor is known as electricity.
  2. Oppositely charged particles attract each other while similarly charged particles repel each other.
  3. In an electric field, the charged particles are deflected towards the oppositely charged plates.
  4. Charged particles are deflected at right angle to the line joining the poles of magnet.
  5. A sharp beam of charged particles can be obtained by using hole or slit.
  6. As the charged particles are not visible so their presence can be revealed by using a photographic plate or film or a fluorescent material.
  7. The charged particles produce fluorescence on striking the fluorescent material.

Discussion of Electric Discharge through Gases at Low Pressure:

Discovery of Electron:

A British scientist, William Crooks, used a special glass tube, for passing electric current through gases. This tube is known as “Discharge tube” or “cathode ray tube ". A cathode ray tube is glass tube having two electrodes inside it at its opposite ends. A small thin tube is present at its one end which is connected to a vacuum pump.

Gas Discharge Tube:

When a gas at ordinary pressure (760mm of Hg) is present inside the tube and then electrodes are connected to the opposite terminals of a battery, then no current passes through the gas. However when the pressure of the gas inside the tube is reduced to 0.1 mm of Hg, with the help of vacuum pump, then the conduction of electricity starts and the gas, present inside the tube begins to glow.
When the pressure of the gas inside the tube is reduced further to 0.01 mm of Hg, while conduction of electricity continues, then the glow disappears and a faint yellowish ray appears which produces fluorescence on the glass, opposite to cathode. Thus it is proved that some rays are going out of cathode towards anode, therefore these rays are called as “cathode rays ". Later on, it was recognized that these rays are actually beams of negatively charged particles. These particles were then given the name “Electron ".

Nature of Cathode Rays (Characteristics of Cathode Rays):

Various scientists studied the nature of cathode rays and pointed out various facts about the nature of cathode rays. These facts about the nature of cathode rays are presented in the form of characteristics of cathode rays as follow.

  1. In 1869, Hittorf observed that if an opaque object is placed in the path of cathode rays, its shadow is formed on the fluorescent screen, at the anode end.
  2. It proves that some radiations (cathode rays) are traveling from cathode towards anode.
  3. Cathode rays always travel in straight path.
  4. When cathode rays strike against the surface of glass, fluorescence is produced.
  5. Crooks in 1870, demonstrated that the cathode rays are actually streams of particles, possessing kinetic energy and momentum. He placed a light pen wheel inside the tube. When cathode rays struck against the paddles of the wheel, it moved towards the anode. Then he exchanged the positions of anode and cathode,. Now it was seen that the paddle moved in the opposite direction. Thus from this experiment Crooks concluded that cathode rays consist of particles which possess K.E and hence momentum.
  6. When cathode rays are passed through an electric field, (inside the discharge tube ), then the particles ( cathode rays ) are deflected towards the positively charged plate of the electric field. The deflection can be observed from the movement of bright spot, formed where cathode rays strike the tube. From this experiment, J. Perin (in 1895) concluded that cathode rays are negatively charged.
  7. The cathode rays heat a foil to incandescence when placed in their way.
  8. When these rays are passed through magnetic field, they deflect at right angle to the magnetic field. It shows that they carry negative charge.
  9. When they strike the surface of a metal (anode), X—rays are produced.
  10. These rays are independent of nature of gas used in the tube. On the basis of all the above mentioned characteristics of cathode ray, it was concluded that these rays actually consisted of negatively charged particles. These negatively charged particles were named as “Electron BY G.J.Stoney in 1891”

Written by: Asad Hussain

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