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SS3 Second Term Physics Lesson Note – Model of the Atom

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WEEK ONE

MODEL OF THE ATOM

Model of the atom
Concept of the atom Rutherford
Bohr
Electron-cloud
Limitations of physical models

Model of the atom

J. THOMSON MODEL

J.J. Thomson proposed an atomic model which visualized the atom as a homogeneous sphere of positive charge inside of which are embedded negatively charged electrons.

He also determined the ratio of the charge to mass, of electrons, and found to be identical for all cathode rays particles, irrespective of the kind of gas in the tube or the metal the electrons are made of.

ERNEST RUTHERFORD MODEL

He proposed a planetary model for the atom which suggested that the atom consists of positively charged heavy core called the nucleus where most of the mass of the atom was concentrated .Around this nucleus, negatively charged electrons circle in orbits much as planets move around the sun. Each nucleus must be surrounded by a number of electrons necessary to produce an electrically neutral atom

LIMITATION OF RUTHERFORD MODEL

It predicts that light of a continuous range of frequencies will be emitted whereas experiment shows line spectra instead of continuous spectra.
It predicts that atoms are unstable-electrons quickly spiral into the nucleus but we know that atoms in general are stable, since the matter around us is stable.

Clearly Rutherford’s model was not sufficient to explain experimental observations. Some sort of modification was needed and this was provided by Neils Bohr.

THE NIELS BOHR MODEL

He suggested a model of hydrogen atom in which:

The orbit at which an electron will move without radiating energy such that its angular momentum is quantized. He called the possible orbits stationary states. Only orbits of particular radii were possible. This orbit is given by the equation:

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