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The introduction of atomic structure.
CHEMISTRY - Secondary 2
In This part we will study:
5. Thomson's model of the atom (1897):
The scientist Thomson carried out many experiments on the electric discharge through gases from which he had discovered the cathode rays.
Discovery of cathode rays (1897):
• It was known that gases don't conduct electricity under normal conditions of pressure and temperature.

• However, gases conduct electricity in a discharge tube whose two poles are connected to an electric source of a suitable high potential difference and under a very low pressure.

• If the potential difference between the two poles of the discharge glass tube increases to around 10000 volts, a stream of invisible rays will be emitted from the cathode (the negative pole), causing a fluorescent glow on hitting the tube wall.
These rays were named "cathode rays".

• It was later known that these rays are composed of minute particles named "electrons".

Properties of cathode rays:
 They consist of very fine negatively charged particles (electrons).
 They move in straight lines (where its speed is lower than the speed of light).
 They have a thermal effect.
 They are affected by both electric and magnetic fields.
 They don't differ either in behavior or in nature if the material of the cathode or the used gas is changed, this is a strong evidence that it is a fundamental constituent of any matter.
In the light of the electrical discharge experiment, Thomson suggested a new atomic model.

The postulate of Thomson's model
He considered the atom as a solid sphere of uniform positive electric charges in which a number of negatively charged electrons is embedded to make the atom electrically neutral.

6. Rutherford's model of the atom (1911):
Rutherford's students Geiger and Marsden performed his famous laboratory experiment.
Rutherford's experiment
The used tools:
• A deep lead box containing a source of alpha particles (a) inside it.
• A metal sheet covered from the inside with a layer of zinc sulphide ZnS.
• A very thin gold foil.
The procedure:
 Alpha particles were allowed to collide the metal sheet, where it was possible to define the location and number of alpha particles by counting the flashes which appeared on the metal sheet.
 A very thin gold foil was placed between the beam of alpha particles and the metal sheet.

The postulates of Rutherford's atomic theory

 Atom:
• It is an extremely small sized particle.
• It has a complicated structure resembles the solar system, since it's composed of a central nucleus (representing the Sun), where the electrons revolve around it (representing the planets).

 Nucleus:
• It is much smaller than the atom and most of the atomic mass is concentrated in it.
• There is a vast space between the nucleus and the orbits of electrons, the atom is not uniformly dense).
• It is positively charged.

 Electrons:
• They have negligible mass compared to that of the nucleus.
• Their charge is negative and equals the nuclear positive charge
(I.e. the atom is electrically neutral).
• They travel around the nucleus at a tremendous speed in special orbits, despite of the mutual attraction between them and the nucleus. This attraction force is equal in quantity and opposite in direction to the centrifugal force resulted from the electron revolving around the nucleus.
• That is why the electron doesn't fall into the nucleus in spite of the attraction force between them, because the attraction force equals in quantity and opposite in direction to the centrifugal force resulted from the revolving of electron around the nucleus.
• Rutherford's theory had failed to explain the atomic structure, because it didn't explain the system in which electrons revolve around the nucleus.

Best wishes to you
Mr.Ahmed Elbasha

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