Wednesday 23 February 2011

Atomic model

The neoclassical view on atomic model
In classical view, matters are continuous and can be cut infinitely many times.
In modern science we have the concept that they are composed by atoms, which is made up of nucleus (containing proton and neutron) and electron.
-          J. J. Thomson suggested that atom is made up of a positively charged sphere with electrons distributed on the sphere. It’s known as plum pudding model. He also contributed to give evidence of the existence of subatomic particles.
-          Rutherford’s atomic model: he suggested that:
1)       Most volume occupied of an atom is empty.
2)       All proton and neutron are concentrated in a small nucleus at the center. They occupied most of the mass. (nucleus is 105 times smaller than the atom)
3)       Negatively charged electrons orbit the nucleus.
Evidence: α particle scattering experiment

α source is emitted and stroke into gold foil. Some of them are deflected and is detected.
The deflected angle is defined by the angle between deflected route and the original route (i.e. route that the particles were not deflected.)
The probability of finding particles at a certain angle is inversely related to the size of angle (most of them are deflected slightly only)
1)       When α particles are able to pass through the gold foil, that implies that the gold atoms has a large number of empty space.
2)       α particles are deflected due to the electric repulsive force between α particle which has charge +2e, and the nucleus. If the α particle goes nearer, it will deflect more.
3)       Large deflection is impossible for the plum pudding model. Thus Rutherford’s model is better to describe an atom.
Limitations of Rutherford’s model
1)       In classical EM theory, accelerating charged particles (electron) will emit radiation and loss energy, which cause the reduction in orbiting radius which is contradictory to the reality.
2)       Consider electron orbiting the proton, mv2/r = Qq/(4πε0r), v2 = e2/(4πε0mr), U = K.E. + P.E. = mv2/2 + Qq/(4πε0r) = e2/(8πε0r) – e2/(4πε0r) = - e2/(8πε0r) < 0 which is impossible.
3)       Atoms only emit radiation at specified frequencies; this can’t be explained by Rutherford’s model.

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