# NCERT Class-9 Science Solutions: Chapter 4-Structure of the Atom Part 8 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 39:

In what way is the Rutherford՚s atomic model different from that of Thomson՚s atomic model?

Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits. There is a positively charged center in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centered in the nucleus. Whereas, Thomson proposed the model of an atom to be similar to a christmas pudding. The electrons are studded like currants in a positively charged sphere like Christmas pudding and the mass of the atom was supposed to be uniformly distributed.

Question 40:

What were the drawbacks of Rutherford՚s model of an atom?

The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo an acceleration and the charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know

Question 41:

What are the postulates of Bohr՚s model of an atom?

The postulates put forth by Neils Bohr՚s about the model of an atom:

1. Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
2. While revolving in discrete orbits the electrons do not radiate energy. These orbits are called energy levels. Energy levels in an atom are shown by circles.

These orbits are represented by the letters K, L, M, N, … or the numbers, n = 1, 2,3, 4, … .

Question 42:

Show diagrammatically the electron distributions in a sodium atom and a sodium ion and also give their atomic number.