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.

Difference between Rutherford՚S Atomic Model and th …

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.


Result for the Electron Distributions in a Sodium Atom and a …

Since the atomic number of sodium atom is 11, it has 11 electrons. A positively charged sodium ion is formed by the removal of one electron from a sodium atom. So, a sodium ion has electrons in it. Thus, electronic distribution of sodium ion will be 2,8. The atomic number of an element is equal to the number of protons in its atom. Since, sodium atom and sodium ion contain the same number of protons, therefore, the atomic number of both is 11.

Question 43:

In the Gold foil experiment of Geiger and Marsden, that paved the way for Rutherford՚s model of an atom, of the were found to deflect at angles . If one mole of were bombarded on the gold foil, compute the number of α-particles that would deflect at angles less than .