Structure of the atom : Rutherford Model of an Atom

 

Rutherford’s Experiment 

Ernest Rutherford a student of Sir J. J. Thomson observed something extraordinary during an experiment using gold foil which disproved his Teacher Thomson’s model.

Rutherford was studying the properties of Radioactive radiations (mainly α alpha rays) for which he had set up some arrangements. 

  1. For his experiments Rutherford used a gold foil (thickness 100nm); over which the stream of α particles were bombarded.
  2.  A radioactive source was used to produce α particles.
  3.  A  zinc sulphide screen was kept covering entire gold foil . ZnS screen has property of fluorescent; means if a massive particle collide with it , it produces luminescence or brightness. 

 

Rutherford made certain observations that contradicted Thomson’s atomic model.

 

S.No. Observations made  Conclusions made by Rutherford
1. A major fraction (99.9%) of the α-particles bombarded towards the gold sheet passed through it without any deflection most of the space in an atom is empty
2. Some of the α-particles (0.09%)were deflected by the gold sheet by very small angles The atom contains positive charge
3. Very few of the α-particles were deflected at angle of 90º or more  the positive charge is concentrated
4. 1 in 20000 or less α – particles retraces its path; means one α – particle came back to the same path with deviation of nearly 180º. The positive charge and mass is concentrated

 

 

Based on the above observations and conclusions, Rutherford proposed the structure of atom.

According to Rutherford’s atomic model:

  1. The positively charged particles and almost entire mass  of an atom are concentrated in an extremely small volume. He called this region of the atom as a Nucleus.
  2. Rutherford’s model proposed that the negatively charged electrons surround the nucleus of an atom.
  3.  To make atom stable, it was suggested that electrons revolve around the nucleus with very high speed in circular paths. He named these circular paths as Orbits.
  4. Electrons being negatively charged and nucleus being a densely concentrated mass of positively charged particles are held together by strong electrostatic force of attraction. To balance this attraction opposite direction centrifugal force exists which prevents electron to fall inside the Nucleus.

 

Limitations of Rutherford Model

Although Rutherford’s atomic model was based on experimental observations; but it failed to answer certain questions.

  • Rutherford proposed that the electrons revolve around the nucleus in fixed paths called Orbits. According to Maxwell theory in Physics, accelerated charged particles emit electromagnetic radiations and hence an electron revolving around the nucleus should emit electromagnetic radiation. This radiation would carry energy from the motion of the electron which would come at the cost of shrinking of orbits. Ultimately the electrons should collapse into the nucleus.  So Rutherford’s model was not in accordance with Maxwell’s theory and could not explain the stability of an atom. (Note: Any charged particle if revolves in circular path, even with same speed, it is changing its velocity; hence accelerating.)
  • The model gives no idea about the arrangement of electrons inside an atom .

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Structure of the atom : Bohr’s Model of an Atom