Chemistry Class IX Chapter "Atomic Structure"

Chapter: Structure of Atom IX CBSE
1. Charged particles in matter
2. The structure of an atom.
3. Thomson's model of an atom.
4. Rutherford's model of an atom
5. Drawbacks of Rutherford’s model of an atom.
6. Charged particles in matter
7. The structure of an atom.
8. Thomson's model of an atom.
9. Rutherford's model of an atom
10. Drawbacks of Rutherford’s model of an atom.
11.Present concept of an atom (Bohr‟s Concept)
12. Bohr‟s model of an atom.
13. Discovery of Neutrons.
14. How are Electrons Distributed in different Orbits (shells).
15. To draw the electronic configuration of an atom of an element.
16. Concept of Valence electrons and Valency.
17. Atomic number and mass number.
18. Isotopes, Isobars and their examples.
19. Fractional atomic weight of the isotopes.
20. Applications of Isotopes.


Q. Write two differences between isobars and isotopes.
Ans: Isobars         
(i) Isobars have the same mass number but different atomic numbers
(ii) Isobars have different chemical properties
Isotopes
Isotopes have the same atomic number but different mass numbers.
Isotopes have similar chemical properties.
Q. Why do isotopes shows similar chemical properties.
Ans: Due to same number of electron in valance shell
Q. How Rutherford concluded that the center part of any atom not empty?
Ans: When Rutherford experimented with alpha particles, then he realized that few particles bounced back directly. Rutherford’s result lead him to believe that most of the foil was made of empty space, but had extremely small, dense lumps of matter inside, which is present only at the center because from center, few particles bounced back. All other particles deflected at different angles. So Rutherford concluded that the center part of any atom is nucleus not empty.
Q. Why an atom of an element is very stable even electrons are in continuous motion?
Ans: The electron could revolve around the nucleus in certain orbits called energy shells and each orbit have different radius. The electrons which are close to nucleus have low energy and the electrons which are farther from the nucleus have high energy .Due to this the electrons do not loose energy and remain stable.
Q. What was the limitation of J .J. Thomson’s model of an atomic?
Ans: When an electron moves around a nucleus, it must continuously radiate out energy and hence, gradually move towards the nucleus in a spiral path, till it collides with the nucleus. Hence atoms are unstable.
Q. When a gold sheet is bombarded by a beam of Alpha particles, only a few of them get deflected whereas most go straight undeflected. Why?
Ans: This is because the nucleus has much smaller volume than that of an atom. The size of atom is 10-10m and size of nucleus is 10-15 m.
Q On the basis of Thomson’s model explains how the atom is neutral.
Ans: J.J. Thomson believed that an atom is made up of positively charged substance in the form of a sphere. Electrons are embedded same way as the apples are embedded in an apple pie. Furthermore, the total positive charge of the sphere is equal to total negative charge of the electrons and hence the atom remained electrically neutral.
Q.Why did Goldstein give the name canal rays to rays that travel from anode to that cathode
Ans: This is because these rays cross the canals of the cathode and reach the other side
Q.What is the limitations of Thomas atomic model?
Ans: Thomson’s model could successfully explain the electrical neutrality of atom. However, it failed to explain how the positively charged particles are shielded from the negatively charged electrons without getting neutralized.

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