Question

The plum pudding ( first model )of the atom was proposed by JJ Thomson. Which of the following is/are the limitations of the Thomson model

(i) The model wasn't able to explain the alpha scattering

(ii) The model failed to explain the position of nuclei 

(iii) The model failed to explain how the positive charges hold the negative charges

(iv)The model failed to explain the existence of electrons in an atom 

1. Statement iv only 
2. Statements I,II&III only
3. Statements I and iii only
4. All the above

Answer 1

Correct Answer - Option 2 : Statements I,II&III only

CONCEPT: 

• The first model of the atom was proposed by JJ Thomson in 1898
• According to this model of the atom
  • In an atom, the positive charges are uniformly distributed and the electrons are embedded in that distribution
  • The whole atom is electrically neutral since the number of positive charges and negative charges is equal
• The limitations of this model are
  • ​Failed to explain the alpha scattering
  • Failed to account for the exact position of the nucleus
  • Failed to explain how the positive charges hold the negatively charged electrons in the distribution of positive charge

EXPLANATION:

• According to the plum pudding model of the atom, the electrons are embedded in the distribution of positive charges.
• The major limitations of the model are
  • According to this model, the positive charges are distributed over the volume of the atom and the electrons are embedded in it.
  • Since the positive charges are distributed over a volume if the alpha particle comes nearby the nucleus it must be deflected.
  • Instead of that in practice during the scattering, most of the alpha particles don't pass without deviation. Hence using this model the alpha particle scattering cannot be explained. Hence statement 1 is a limitation.
  • Since most of the alpha particle passes without any deviation most part of the atom must be empty or the positive charges must be concentrated to a small volume (i.e., Nucleus ). This is contrary to the argument of the plum pudding model, which means the model wasn't able to account for the exact position of the nucleus. Hence statement 2 is a limitation
  • The model also fails to explain how the positive charges hold the negative charges. Hence statement 3 is a limitation of the model.
  • The electrons were invented by JJ Thomson. The existence of electrons was earlier proved before this model. Hence statement 3 is not related to the model
  • The only statements I, II, and III are limitations. Hence option 2  is the answer