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An atom consists of Z electrons is embedded in a cloud of positive charges that exactly neutralise
that of the electrons’. The positive cloud is heavy and comprising most of the atom’s mass. Inside
a stable atom, the electrons sit at their respective equilibrium position where the attraction of the
positive cloud on the electrons balances the electron’s mutual repulsion.

Rutherford shots some alpha particles (which are really just the nucleus of a helium atom) at
some really thin gold foil. Rutherford found that most of alpha went right through the foil and
some of them did bounce back. Rutherford’s experiment prompted a change in the atomic
model. Based on his experimental evidence:
o The atom is mostly empty space
o All the positive charge, and almost all the mass is concentrated in a small area in the center.
o He called this a “nucleus”
o The nucleus is composed of protons and neutrons
o The electrons distributed around the nucleus and occupy most of the volume
o His model was called a “nuclear model”

The large deflection of alpha particle as seen in the scattering experiment with a thin gold foil
must be produced by a close encounter between the alpha particle and a very small but massive
kernel inside the atom. In contrast, a diffused distribution of the positive charge as assumed in
plum-pudding model cannot do the job. Plum-pudding model by Thompson, it fails to explain
the emission and absorption line spectrum from atoms because it predicts only a single
emission frequency. Most importantly it fails to explain the back-scattering of alpha particle
seen in Rutherford’s scattering experiment because the model predicts only.

According to classical EM, the Rutherford model for atom (a classical model) has a fatal flaw:
it predicts the collapse of the atom within 10-10 s. An accelerated electron will radiate EM
radiation, hence causing the orbiting electron to loss energy and consequently spiral inward
and impact on the nucleus. The Rutherford model also cannot explain the pattern of discrete
spectral lines as the radiation predicted by Rutherford model is a continuous burst.

(30 Marks)


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