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|The Discovery of Nucleus by Rutherford : Author - Ms. Dhanya Nair|
The discovery of electron by J.J Thomson demonstrated for
the first time that atoms were further divisible into tiny parts.
During the same period Curies studied an interesting phenomenon called radioactivity and found that some heavy elements like Uranium, Radium etc. are capable of emitting radiationconsisting of charged particles. This phenomenon gave substantial evidence that atoms were made up of tiny positively and negatively charged particles.
Lord Rutherford later on while studying the nature of these radiation discovered that the radioactive elements emit three types of radiations with different penetrating power namely alpha (least penetrated), beta (moderately penetrating) and Gamma (highly penetrating). When these radiations were subjected to electric fields, alpha rays were found to be positively charged particles while beta rays were found as negatively charged light particles. Gamma rays were found to be neutral in nature.
While studying the properties of these radiations, Rutherford noticed that the beam of alpha rays when were made to fall on a photographic plate by passing it through air, the edges of the slit became diffused and widened. He concluded that the alpha rays were scattered by air molecules.
To get a clearer evidence about the scattering he along with his coworkers
Geiger and Marsden built an apparatus to bombard alpha particles on
thin metallic foils. The metallic foil he selected for his experiment
was a very thin gold foil.
Now if the atoms were actually arranged according to Thomson, then the alpha particles would go undeflected. (Since the electrons are too tiny to deflect the massive alpha particles while the deflection caused due to repulsive force acting between the positively charged particles and the alpha particles is very less).
But during experimentation it was found that though most of the alpha particles went right through the thin foil, a few alpha particles rebounded almost directly backwards. These large angle deflections were not consistent with Thomson's model.
From the above observations he concluded that large
On the basis of these postulates he was able to explain the -scattering
experiment, most particles pass directly through the foil because they
do not encounter the minute nucleus; they merely pass through the empty
space of the atom. Occasionally a particle comes into the close vicinity
of a gold nucleus, however. The repulsion between the highly charged
gold nucleus and the particle is strong enough to deflect the less massive
Problems faced by Rutherford's Atomic model:
Rutherford's atomic model was unstable:
This radiated energy would be at the expense of the
electrostatic potential energy of the electron. The reducing potential
energy increases the electrostatic force of attraction between the
nucleus and the electron. As the energy of the electron decreases,
it can no longer maintain its orbit, so it spirals in toward the proton.
The frequency of the wave should be the same as the number of orbits
that the electron makes each second. As the electron spirals in, it
orbits faster and faster, generating electromagnetic waves of higher
and higher frequency, until finally, it crashes into the proton itself
and the atom collapses i.e. the atom vanishes in a fraction of second.
Since Rutherford's atomic model failed to explain the unique spectral properties of the elements and their chemical properties and moreover the stability of the atoms, the model was discarded.
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