Structure of Atom - II

 1.  Charge on an electron was determined by R A Millikan by using an oil drop experiment. The value of the charge on an electron is -1.6 x 10^-19C.

2.  The mass on an electron was determined by combining the results of Thomson’s experiment and Millikan’s oil drop experiment. The mass of an electron was determined to be 9.1094 x 10^-31kg.


3. Discovery of protons and canal rays: Modified cathode ray tube experiment was carried out which led to the discovery of protons.

4. Characteristics of positively charged particles:
a. Charge to mass ratio of particles depends on gas from which these originate
b. The positively charged particles depend upon the nature of gas present in the cathode ray discharge tube
c. Some of the positively charged particles carry a multiple of fundamental of electrical charge.
d. Behaviour of positively charged particles in electrical or magnetic field is opposite to that observed for cathode rays

5. Neutrons were discovered by James Chadwick by bombarding a thin sheet of beryllium by α- particles. They are electrically neutral particles having a mass slightly greater than that of the protons.


6. Atomic number (Z) : The number of protons present in the nucleus (Moseley 1913).


7. Mass Number (A) : Sum of the number of protons and neutrons present in the nucleus.


8. Thomson model of an atom: This model proposed that atom is considered as a uniform positively charged sphere and electrons are embedded in it. An important feature of Thomson model of an atom was that mass of atom is considered to be evenly spread over the atom.Thomson model of atom is also called as Plum pudding, raisin pudding or watermelon model Thomson model of atom was discarded because it could not explain certain experimental results like the scattering of α- particles by thin metal foils.


9. Observations from α- particles scattering experiment by Rutherford:
a. Most of the α- particles  passed through gold foil un deflected
b. A small fraction of α- particles got deflected through small angles
c. Very few α- particles did not pass through foil but suffered large deflection nearly 180^o

10. Conclusions Rutherford drew from α- particles scattering experiment:
     a. Since most of the α-particles passed through foil undeflected, it     means most of the space in atom is empty

     b. Since some of the α-particles are deflected to certain angles, it means that there is positively mass present in atom

     c. Since only some of the α-particles suffered large deflections, the positively charged mass must be occupying very small space

     d. Strong deflections or even bouncing back of α-particles from metal foil were due to direct collision with positively charged mass in atom

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