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WEEK 8
TOPIC: ATOMIC THEORY
CONTENT:
- Gross features of the atom – J.J. Thompson’s experiment
- Bohr’s model of the atom,
- Outline description of the Rutherford’s alpha scattering experiment.
PERIOD 1: J.J. THOMPSON’S CATHODE RAYS EXPERIMENT
Experiments on discharge tubes performed by J.J Thomson (1897) led to the discovery of the electron (cathode ray) as a subatomic particle .Cathode rays:
- move in a straight line normal to the cathode and can cast the shadow of an object placed along their path.
- possess kinetic energy and so can cause the motion of a mechanical paddle wheel placed along their path.
- attract positive charge and repel negative charge, indicating that they are negatively charged.
- have a constant value of 1.76 x coulomb for the charge –to-mass ratio () no matter the gas used in the tube or the nature of the materials of the electrodes and the tube. This indicates that electrons are fundamental particles of all atoms and they are indistinguishable i.e., the universality of electrons in atoms.
PERIOD 2: BOHR’S MODEL OF THE ATOM
Niels Bohr (1913) put forward his own model of the atom based on the quantum mechanics originally developed by plank. He assumed the Rutherford’s model and suggested that the extranuclear part consists of electrons in specific spherical orbits (shells or energy levels)
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