![Why is the classical(Rutherford)
model for an atom of electron orbiting
around the nucleus not able to explain
the atomic structure?[Delhi 2012]](https://image.slidesharecdn.com/rutherfordmodelalphaparticlescattering-200930171033/85/Rutherford-model-alpha-particle-scattering-16-320.jpg)
Rutherford model alpha particle scattering
- 2. Content ● Discovery of atom ● Experimental apparatus ● Expectations Vs Result ● Distance of closest approach ● Rutherford’s Atom Model ● Drawbacks of Rutherford’s Model ● Q&A
- 3. The first contribution in this regard came from Dalton who postulated that matter is made of atoms, which are indivisible. The word atom comes from a Greek word Atmos which means ‘indivisible’. J.J.Thomson proposed a structure for the atom, named as plum pudding model which was modified by Rutherford and later by Niels Bohr.
- 4. Experimental arrangement Source: Alpha-Particles are emitted by some radioactive material(Bismuth) kept inside a thick lead box Collimator: Lead bricks were used to collimate into a narrow beam of alpha-particles to the foil Foil: The beam was allow to fall on a thin foil of gold of thickness 2.1*10^-7m Screen: The scattered alpha-particles hit the Zinc Sulphide screen Detector:Microscope is used to detect which is moved to all 360 degrees Medium: The whole setup was kept inside a vacuum chamber
- 5. What is an alpha-particles? Why are they used in this experiment? ● Heavy particles with positive charge ● It is the nuclei of helium atom which is made of 2 protons and 2 neutrons ● They are 7300 times bigger than electron ● High penetrating power with a speed of 5.5 MeV
- 7. setup
- 9. Experimental data points obtained by Geiger and Marsden
- 10. Atom consists of a central heavy nucleus containing positive charge and negatively charged electrons circulate around the nucleus in circular orbits. Rutherford model could explain the an neutrality of an atom, thermionic emission and photoelectric effect;but is could not explain the stability of an atom and the observed line spectrum of an atom(atomic spectrum). Rutherford’s Atom Model
- 11. Why did he named it planetary model?....
- 12. Impact parameter is defined as the perpendicular distance of the velocity vector of alpha particle from the centre of the nucleus. Angle of scattering is defined as the angle between the direction of approach and the direction of recede of the alpha particle. Distance of closest approach is the centre-to-centre distance between the alpha particle and the gold nucleus when the alpha particle is at its stopping point. Alpha particle trajectory
- 13. Distance of closest approach
- 14. The radius of its orbit would go on decreasing and it would finally spiral into the nucleus, resulting in an ultimate catastrophic collapse of the atom as shown in figure Failed to explain the stability
- 15. As an electron can revolve in any orbit, according to em theory, it must emit continuous rediations of all wavelengths as shown in the figure. Actually the elements are found to emit spectral lines of definite frequencies and not all of them. Fail to explain line spectrum
- 16. Why is the classical(Rutherford) model for an atom of electron orbiting around the nucleus not able to explain the atomic structure?[Delhi 2012]
- 17. Ans. The classical method could not explain the atomic structure as the electron revolving around the nucleus are accelerated and emits energy as the result, the radius of the circular paths goes on decreasing. Ultimately electrons fall into the nucleus, which is not in practical.
- 18. In a Geiger-Marsden experiment, calculate the distance of closest approach to the nucleus of Z=75, when an Alpha particle of 5 MeV energy impinges on it before it comes momentarily to rest and reverses its direction.
- 20. How will the distance of closest approach be affected when the kinetic energy of the alpha particle is doubled?
- 21. When the kinetic energy is doubled the distance of closest approach is halved.