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Atomic structure



ATOM : atom is the smallest indivisible particle of the matter. Atom is made of electron, proton and neurons.

Electrons were discovered using cathode ray discharge tube experiment. Nucleus was discovered by Rutherford in 1911.

Cathode ray discharge tube experiment: A cathode ray discharge tube madeof glass is taken with two electrodes. At very low pressure and high voltage,current starts flowing through a stream of particles moving in the tube fromcathode to anode. These rays were called cathode rays. When a perforatedanode was taken, the cathode rays struck the other end of the glass tube atthe fluorescent coating and a bright spot on the coating was developed

Results of Rutherford experiments:

  • Cathode rays consist of negatively charged electrons.
  • Cathode rays themselves are not visible but their behavior can be observed with help of fluorescent or phosphorescent materials
  •  In absence of electrical or magnetic field cathode rays travel in straight linesd. In presence of electrical or magnetic field, behaviour of cathode rays is similar to  that shown by electrons
  •  The characteristics of the cathode rays do not depend upon the material of the electrodes and the nature of the gas present in the cathode ray tube.

Charge to mass ratio of an electron was determined by Thomson. The chargeto mass ratio of an electron as 1.758820 x 1011. C kg-1  Charge on an electron was determined by R A Millikan by using an oil dropexperiment. The value of the charge on an electron is -1.6 x 10-19C.

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

Discovery of protons and canal rays:

Modified cathode ray tube experimentwas carried out which led to the discovery of protons.

 Characteristics of positively charged particles:
  •  Charge to mass ratio of particles depends on gas from which these originateb. The positively charged particles depend upon the nature of gas present in the cathode ray discharge tube
  •  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

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.

Atomic number (Z) : the number of protons present in the nucleus(Moseley1913).

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

Thomson model of an atom:

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

Observations from α- particles scattering experiment by Rutherford :

  •  Most of the α- particles passed through gold foil un deflected
  • A small fraction of α- particles got deflected through small angles
  •  Very few α- particles did not pass through foil but suffered large deflection nearly180o

Conclusions Rutherford drew from α- particles scattering experiment:

  •  Since most of the α-particles passed through foil undeflected, it means most of the  space in atom is empty
  •  Since some of the α-particles are deflected to certain angles, it means that there is positively mass present in atom
  •   Since only some of the α-particles suffered large deflections, the positively charged mass must be occupying very small space
  • 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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