Class 11 Chemistry Structure of Atom Atomic Models Of Atom



In 1904, the attempt to explain structure of Atom was done by J.J Thomson.


According to Thomson :Atom is positively charged sphere in which few negative charges are scattered at certain places like plums in pudding or cherries in ice creams.


Limitation of this model: However, this model failed. The reason about failure was as it wasn’t possible that few negative charges remain scattered in the positive sphere. As we know, opposite charges attract each other. So, these negative charges would have been neutralized. They can’t remain scattered. However, this couldn’t be explained by Thomson therefore the model failed.



In order to explain the structure of atom, Rutherford performed the scattering experiment.

For this, he took Gold foil and passed Alpha rays through it. The source of these Alpha rays was radioactive substance which was kept in a Lead block (as Lead is inert towards it). These rays were made to pass through a slit. Alpha rays are actually positively charged rays consisting of Helium nucleus (He). To notice the scattering of rays, he fitted movable screen made of florescent material around the gold foil.



Following observations were made: -

  1. Some rays passed straight.
  2. Some rays were deflected through small & large angles.
  3. Some rays rebound back.



Conclusion drawn:

  • Most of the space in atom is empty.
  • There is something in center of an atom called Nucleus.
  • Nucleus is +vely charged.

So, According To Rutherford ,structure of an atom is similar to solar system.     



He said,

  • Atom is electrically neutral.
  • Nucleus is in center.
  • In which protons & neutrons are present.
  • Outside nucleus electrons revolve like planets revolve around sun.

He said, atom is made of two parts: nucleus and extra nuclear part .

In Nucleus, positive charge is present and in extra nuclear part electrons are present as shown in figure:

Proton  -> (+)vely charged 

Electron  -> (-)vely charged 

Neutron  -> No charge

Limitation : he failed to explain the stability of an atom”.

          According To Electromagnetic theory, a charged particle revolving in circular path continuously emits energy and shorten its path.


As we know, Electrons are also a charged particle revolving in circular path. So, they should also emit energy and shorten its path .As a result, they should finally fall into nucleus. But this doesn’t happen.

Some definitions were also given related to atom:

Atomic number: It is equal to number of protons in an atom .It is represented by ‘Z’and is written as subscript.

Atomic Mass: It is equal to number of nucleons, or we can say sum of protons and neutrons in nucleus of an atom. It is represented as ‘A’ and is written as superscript.

Representation of atom of an element

It is the method of representation of element by specifying its symbol, atomic number and mass number .The rule involved is  writing symbol and specifying its atomic number as subscript and mass number as superscript.


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