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Rutherford’s Atomic Model

  • Last Updated : 21 Sep, 2021
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Since J.J. Thomson’s atomic model was flawed and had significant drawbacks, a newer model was proposed by Ernest Rutherford which served as the base for the following atomic models. In search of answers regarding the arrangement of electrons within an atom, Rutherford conducted an experiment in which fast-moving alpha particles struck a thin gold foil.

Rutherford’s Alpha (α) Particle Scattering Experiment

To determine how electrons are arranged in an atom, the Alpha (α) Particle Scattering Experiment was organized by Rutherford. Rapidly moving α-particles were directed to bombard a thin sheet of gold.

  • The gold foil was selected so as to obtain an extremely thin layer. The thickness of the gold foil was about 1000 atoms.
  • Doubly-charged helium ions are known as α-particles. Rapidly moving α-particles possess a great deal of energy, as they have a mass of about 4 amu.

The hypothesis was that α-particles would be deflected by the sub-atomic particles in the gold atoms. Rutherford didn’t expect to witness significant deflections as the α-particles were considerably heavier than the protons. However, the experiment produced entirely unanticipated results.

Observations

Rutherford observed the following from his α-particle scattering experiment:



  1. The majority of the fast-moving α-particles went directly through the gold foil.
  2. The foil deflected some of the α-particles by fairly small angles.
  3. Only a few α-particles were completely deflected back (by 180 degrees).

Figure 1. Rutherford’s Alpha Particle Scattering Experiment (above)

Conclusion

Rutherford concluded the following from his observations:

  1. Because a large proportion of the α-particles directed towards the gold sheet went through it without any deflection, so, majority of the space in an atom is vacant.
  2. Only a few α-particles were diverted off their route, suggesting that the atom’s positive charge takes up relatively little space.
  3. Since a very tiny percentage of α-particles completely rebounded, this implied that the atom’s mass and positive charge are concentrated in a small volume and not uniformly distributed.

Rutherford’s Model of the Atom

Rutherford model of atom is also known as the Nuclear Model of Atom.

Features of Rutherford’s Model of the Atom:

Rutherford hypothesized the atomic structure of elements based on the observations and conclusions stated above. The Rutherford atomic model states:

  1. Positively charged particles and the majority of an atom’s mass were concentrated in an incredibly tiny volume. He referred to this portion of the atom as the nucleus. This nucleus is very small in size, as compared to the size of the atom as a whole.
  2. According to Rutherford, negatively charged electrons encircle the nucleus of an atom. He believed that the electrons encircling the nucleus travel around it in circular routes at great speeds. He referred to these circular routes as well-defined orbits.

The nucleus and electrons were held together by electrostatic forces of attraction.

Figure 2. Rutherford’s Model of the Atom (above)

Drawbacks of Rutherford’s Model of the Atom

  1. Rutherford’s Model predicts that electrons will orbit around the positively charged nucleus, which is not anticipated to be stable. A charged particle in rapid motion along a circular route, would lose energy continually and eventually collapse into the nucleus. This causes an atom to be unstable, whereas we know that atoms are extremely stable.
  2. Because it merely postulated the existence of protons in the nucleus, the Rutherford Model could not resolve the problem of atomic mass.

Figure 3. The drawback of Rutherford’s Model of the Atom 

Sample Questions

Question 1: Why was J.J. Thomson’s atomic model flawed?



Answer:

Thomson’s atomic model does not explain how the positive charge on the electrons inside the atom is maintained. It also fails to explain the stability of an atom. The nucleus of an atom is not mentioned in the hypothesis. It could not explain Rutherford’s scattering experiment.

Question 2: Briefly explain Rutherford’s scattering experiment.

Answer:

To determine how electrons are arranged in an atom, the Alpha (α) Particle Scattering Experiment was organized by Rutherford. Rapidly-moving α-particles were directed to bombard a thin sheet of gold. The gold foil was selected so as to obtain an extremely thin layer. The thickness of the gold foil was about 1000 atoms. Doubly-charged helium ions are known as α-particles. Rapidly-moving α-particles possess a great deal of energy, as they have a mass of about 4 amu.

The hypothesis was that α-particles would be deflected by the sub-atomic particles in the gold atoms. Rutherford didn’t expect to witness significant deflections as the α-particles were considerably heavier than the protons. However, the experiment produced entirely unanticipated results.

Question 3: Which sub-atomic particle was discovered by Rutherford through his Alpha (α) Particle Scattering Experiment?

Answer:

The sub-atomic particle discovered by Rutherford through his Alpha (α) Particle Scattering Experiment was the Nucleus.

Question 4: Why was a gold foil used in the Alpha (α) Particle Scattering Experiment?



Answer:

A gold foil was selected so as to obtain an extremely thin layer and as it is the most malleable metal. However, if any other metal foil was used, the results obtained would be consistent.

Question 5: What was the main drawback of Rutherford’s Model of the Atom?

Answer:

Rutherford’s Model predicts that electrons will orbit around the positively charged nucleus, which is not anticipated to be stable. A charged particle in rapid motion along a circular route, would lose energy continually and eventually collapse into the nucleus. This causes an atom to be unstable, whereas we know that atoms are extremely stable.

Question 6: By which angles did the α-particles get deflected in the scattering experiment?

Answer:

The majority of the fast-moving α-particles went directly through the gold foil. The foil deflected some of the α-particles by fairly small angles. Only a few α-particles were completely deflected back (by 180 degrees).

Question 7: How did Rutherford define an Orbit?

Answer:

According to Rutherford, negatively charged electrons encircle the nucleus of an atom. He believed that the electrons encircling the nucleus travel around it in circular routes at great speeds. He referred to these circular routes as well-defined orbits.

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