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(Chemistry Ch-2) 2.Thomsons Atomic Model, Rutherfords Atomic Model, Drawbacks of Rutherfords Model, Atomic Number, Isobars and Isotopes

Thomson’s Model of Atom
  • An atom possesses a spherical shape in which the positive charge is uniformly distributed.
  • Known as plum pudding, raisin pudding or watermelon model
  • Feature: The mass of an atom is assumed to be uniformly distributed across the atom.

Rutherford’s scattering experiment
  • Experimental Setup:
  • A stream of high-energy α-particles from a radioactive source was directed at the thin gold foil.
  • Gold foil had a circular fluorescent screen (of zinc sulphide) around it.
  • A tiny flash of light was observed at the point, whenever an α-particle struck the screen.
  • Observation:
  • Most of the alpha (α) particles passed through the gold foil without deflection.
  • Some of the α-particles (a small fraction) were deflected by small angles.
  • Very few of them were deflected nearly by 180°.
  • Conclusions:
  • Most of the space in an atom is empty as most of the α-particles passed through the gold foil un-deflected.
  • The positive charge is concentrated in a very small volume that repelled and deflected few positively charged α-particles. This small volume of the atom is called nucleus.
  • The volume of the nucleus is negligibly small as compared to the total volume of the atom.
Rutherford’s Model of Atom
Based on his experiment, Rutherford proposed:
  • Positive charge and most of the mass of an atom are densely concentrated in a small region (called nucleus).
  • Electrons revolve around the nucleus, with very high speeds, in circular paths called orbits.
  • Electrons and nucleus are held together in the atom by a strong electrostatic force of attraction.
Drawbacks of Rutherford’s Model
  • It cannot explain the stability of an atom on the basis of classical mechanics and electromagnetic theory.
  • If the electrons were stationary, then the strong electrostatic force of attraction between the dense nucleus and the electrons would pull the electrons towards the nucleus. Thus, it cannot explain the stability of an atom.
  • Rutherford’s model does not give any idea about the distribution of electrons around the nucleus (i.e., the electronic structure of the atom), and about their energy.
  • It cannot explain the atomic spectra.
Atomic Number and Mass Number
  • Atomic number (Z) = Number of protons in the nucleus of an atom
= Number of electrons in a neutral atom
  • Mass number (A) = Number of protons (Z) + Number of neutrons (n)
Isotopes and Isobars
  • Isotopes are the atoms with the same atomic number, but different mass numbers.
For example, and  are the isotopes of hydrogen.
  • Isobars are the atoms with the same mass number, but different atomic numbers.
For example,  and and 


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