Atom is the smallest indivisible particle of the matter. Atom is made of electron,
proton and neutrons.
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
Discovery of protons and canal rays: Modified cathode ray tube
experimentwas carried out which led to the discovery of protons.
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 modelThomson model
of atom was discarded 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:
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
nearly180o.
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