What is the difference in the
models of Thom and Rutherford
to Valandrondomme
Answers
Answer:
Bohr thought that electrons orbited the nucleus in quantised orbits. Bohr built upon Rutherford's model of the atom. In Rutherford's model most of the atom's mass is concentrated into the centre (what we now call the nucleus) and electrons surround the positive mass in something like a cloud.
Answer:
Thomson’s Atomic Model
In 1898, J. J. Thomson proposed the first of many atomic models to come. He proposed that an atom is shaped like a sphere with a radius of approximately 10-10m, where the positive charge is uniformly distributed. The electrons are embedded in this sphere so as to give the most stable electrostatic arrangement.
An important aspect of this model is that it assumes that the mass of the atom is uniformly distributed over the atom. Thomson’s atomic model was successful in explaining the overall neutrality of the atom. However, its propositions were not consistent with the results of later experiments. In 1906, J. J. Thomson was awarded the Nobel Prize in physics for his theories and experiments on electricity conduction by gases.
Explanation:
Rutherford’s Atomic Model
The second of the atomic models was the contribution of Ernest Rutherford. To come up with their model, Rutherford and his students – Hans Geiger and Ernest Marsden performed an experiment where they bombarded very thin gold foil with α-particles. Let’s understand this experiment.
α-Particle Scattering Experiment
Quick summary
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Thomson's model of atom
2 mins read
Rutherford Model
2 mins read
Drawbacks of Rutherford Model
2 mins read
Experiment
In this experiment, high energy α-particles from a radioactive source were directed at a thin foil (about 100nm thickness) of gold. A circular, fluorescent zinc sulfide screen was present around the thin gold foil. A tiny flash of light was produced at a point on the screen whenever α-particles struck it.
Rutherford’s alpha-particle scattering experiment [Source: Wikimedia Commons]
Results
Based on Thomson’s model, the mass of every atom in the gold foil should be evenly spread over the entire atom. Therefore, when α-particles hit the foil, it is expected that they would slow down and change directions only by small angles as they pass through the foil. However, the results from Rutherford’s experiment were unexpected –
Most of the α-particles passed undeflected through the foil.
A small number of α-particles were deflected by small angles.
Very few α-particles (about 1 in 20,000) bounced back.
Thomson’s model versus Rutherford’s model [Source: Wikimedia Commons]
Conclusions of the α-scattering experiment
Based on the above results, Rutherford made the following conclusions about the structure of the atom:
Since most of the α-particles passed through the foil undeflected, most of the space in the atom is empty.
The deflection of a few positively charged α-particles must be due to the enormous repulsive force. This suggests that the positive charge is not uniformly spread throughout the atom as Thomson had proposed. The positive charge has to be concentrated in a very small volume to deflect the positively charged α-particles.
Nuclear Model Of The Atom
Based on his observations and conclusions, Rutherford proposed his model of the structure of the atom. According to this model –
.
Drawbacks Of Rutherford’s Atomic Model
According to Rutherford’s atomic model, the electrons (planets) move around the nucleus (sun) in well-defined orbits. Since a body that moves in an orbit must undergo acceleration, the electrons, in this case, must be under acceleration. According to Maxwell’s electromagnetic theory, charged particles when accelerated must emit electromagnetic radiation. Therefore, an electron in an orbit will emit radiation and eventually the orbit will shrink. If this is true, then the electron will spiral into the nucleus. But this does not happen. Thus, Rutherford’s model does not explain the stability of the atom.
Contrarily, let’s consider that the electrons do not move and are stationary. Then the electrostatic attraction between the electrons and the dense nucleus will pull the electrons into the nucleus to form a miniature version of Thomson’s model.
Rutherford’s model also does not state anything about the distribution of the electrons around the nucleus and the energies of these electrons.