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Structure of Atom(Part 5)

 Rutherford's Atomic Model:

Rutherford performed 'Gold Foil' experiment to understand how negative and positive charges could coexist in an atom. He bombarded alpha particles on a 0.00004 
cm thick gold foil. Alpha particles are emitted by radioactive elements like radium and polonium. These are actually helium nuclei (He2+). They can penetrate through matter to some extent.

He observed the effects of α-particles on a photographic plate or a screen coated
with zinc sulphide. He proved that the 'plum-pudding' model of the atom was not correct. 

Observations made by Rutherford were as follows:
i.Almost all the particles passed through the foil un-deflected.
ii.Out of 20000 particles, only a few were deflected at fairly large angles and very few bounced back on hitting the gold foil.



Results of the experiment:


Keeping in view the experiment, Rutherford proposed planetary model for an atom and concluded following results:

i.Since most of the particles passed through the foil undeflected, therefore most of the volume occupied by an atom is empty. 

ii.The deflection of a few particles proved that there is a 'center of positive charges' in an atom, which is called 'nucleus' of an atom.

iii.The complete rebounce of a few particles show that the nucleus is very dense and hard.

iv.Since a few particles were deflected, it shows that the size of the nucleus is very small as compared to the total volume of an atom.

v.The electrons revolve around the nucleus.

vi.An atom as a whole is neutral, therefore the number of electrons in an atom is equal to the number of protons.

vii.Except electrons, all other fundamental particles that liewithin the nucleus, are known as nucleons.


Defects in Rutherford's Model:

Although Rutherford's experiment proved that the 'plum-pudding' model of an atom was not correct, yet it had following defects:

i.According to classical theory of radiation, electrons being the charged particles should release or emit energy continuously and they should ultimately fall into the nucleus.

ii.If the electrons emit energy continuously, they should form a continuous spectrum but in fact, line spectrum was observed.

 Although the scientists had objections on the atomic model presented by Rutherford, yet it cultivated thought provoking ideas among them. They initiated the quest to answer the following questions:

i.How can an atom collapse or why are atoms stable?

ii.Why does an atom give line spectrum?

iii.Scientists considered there must be another model of atom. It indicated that
Rutherford's model was not perfect.



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