Ii) The size of the nucleus is very small in comparison to the size of the atom. The atom’s central core is called nucleus. I) All the protons (+ve charge) and the neutrons (neutral charge) i.e nearly the total mass of an atom is present in a very small region at the centre of the atom. On the basis of the above observation, and having realized that the rebounding α-particles had met something even more massive than themselves inside the gold atom, Rutherford proposed an atomic model as follows. The fact that one in 20,000 have deflected at 180° backwards indicates that volume occupied by this heavy positively charged body is very small in comparison to total volume of the atom. The fact that few α - particles are deflected at large angles indicates the presence of a heavy positively charge body i.e., for such large deflections to occur α - particles must have come closer to or collided with a massive positively charged body.ģ. The fact that most of the α - partaticles passed straight through the metal foil indicates the most part of the atom is empty.Ģ. There is far less difference between air and bullet than there is between gold atoms and α-particles assuming of course that density of a gold atom is evenly distributed.ġ. Some were even scattered in the opposite direction at an angle of 180°. Very few α–particles are deflected backwards through angles greater than 90°. Some α–particles are deflected from their path and diverge. Majority of the α–particles pass straight through the gold strip with little or no deflection. The zinc sulphide screen gives off a visible flash of light when struck by an α particle, as ZnS has the remarkable property of converting kinetic energy of α particle into visible light. Rutherford allowed a narrow beam of α–particles to fall on a very thin gold foil of thickness of the order of 0.0004 cm and determined the subsequent path of these particles with the help of a zinc sulphide fluorescent screen. Α– particles emitted by radioactive substance were shown to be dipositive Helium ions (He++) having a mass of 4 units and 2 units of positive charge. This model is called the plum – pudding model after a type of Victorian dissert in which bits of plums were surrounded by matrix of pudding. Thomson, in 1904, proposed that there was an equal and opposite positive charge enveloping the electrons in a matrix. Now let us see, how these particles are arranged in an atom to suggest a model of the atom. We know the fundamental particles of the atom.
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