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Saturday, April 9, 2011

The Discovery of Nucleus by Rutherford

Author - Ms. Dhanya Nair

Note : Also check out the higly detailed animation on our site
Discovery of electron by JJ Thomson
Discovery of Nucleus by Rutherford
Millikan's Oil drop experiment

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.

Rutherford expected the atoms to be as suggested by Thomson. According to Thomson's model, atom is a ball of uniformly distributed positive charge with electrons embedded inside the sphere. This model is called as Plum pudding or chocolate chip cookie model.

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).

gold foil experiment of rutherford

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 scattering angles
could only occur if most of the mass of the atom, and all of its positive charge reside in a very small, extremely dense region called the atomic nucleus. Most of the total volume of the atom is empty space in which electrons move around the nucleus.

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 particle.

Thus Rutherford established a new model of the atom that was similar to the structure of planets in orbit around the sun. Rutherford's model consisted of a dense positive core at the center of the atom with electrons orbiting around it. This is called as Planetary Model of the atom.


Though Rutherford was successful enough to explain the rearrangement of the charged particles within the atom, his model faced some serious problems.

Problems faced by Rutherford's Atomic model:
If the model was correct, then one could show that atoms should not exist for more than a fraction of a second. However atoms were always found to be present from time immemorial i.e. Rutherford's atom was found to be unstable.


electron's circular motion

Rutherford's atomic model was unstable:


According to Rutherford's atomic model the electrons revolve around the nucleus in circular orbits. Now an orbiting electron is accelerated due to the centripetal force acting on it. Any accelerated charge particle loses its energy by emitting electromagnetic radiation.

em radiation emission by electron

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.
This means that the atom should emit light of different frequencies, starting in the red (low frequencies) and continuing to the blue (high frequencies) i.e. a continuous band of spectrum before the atom finally collapses but atoms were always found to emit the line spectra.

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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