Q28 of 40 Page 4

Compare all the proposed models of an atom given in this chapter.

A comparison between the models of an atom proposed by J.J. Thomson, Rutherford and Bohr is given below:



     Model  Thomson's model of an atom  Rutherford's model of an atom  Bohr's model of an atom
 Property: atom in which the atom consisted of a positively charged sphere. The electrons are embedded in the sphere. The negative and the positive charges are equal in magnitude. Hence, the atom is electrically neutral. Rutherford put forward the model of an atom which had a positively charged center called nucleus. It also had the electrons which revolved around the nucleus in well defined orbits.

(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom

.(ii) While revolving in discrete orbits the electrons do not
radiate energy.

(iii)The maximum number of electrons present in a shell is given by the formula 2n2

(iv)The maximum number of electrons that can be accommodated in the outermost orbit is 8.
Limitation:  J.J. Thomson's model of the atom could not explain the results of alpha particle scattering experiment carried out by Rutherford the orbital revolution of the electron is not expected to be stable. Any particle which is in a circular orbit would undergo an acceleration. Charged particles would radiate energy during acceleration. Electrons are negatively charged. Thus, the revolving electron would lose energy. It will finally fall into the nucleus. If this was the situation, then the atom should be highly unstable. Hence, matter would not exist in the form that we know. (i) Heisenberg principle - Position and momentum of a particle cannot be determined at same time, accurately. Result of both of them is greater than h/4π.

(ii)In Bohr's theory, orbits were circular but now we know that they are 3-D and not 2-D.

More from this chapter

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26

(a) What is an octet? How do elements reach an octet?

(b) Make a schematic atomic structure of Magnesium of Phosphorus.


(Given: number of protons of Magnesium = 12, Phosphorus = 15).

 27

Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

 29

Complete the following table:

 30

(a) Write the name of the sub – atomic particle discovered by J. Chadwick. What type of charge occurs on this particle? In which part of an atom this particle is located?

(b) List three steps of an experiment performed by Rutherford for his model of an atom.


(c) Define isobar, write its one example.


(d) Which scientist concluded that the size of the nucleus is very small as compared to the size of an atom?