Sunday, December 2, 2012

Structure of atom for class 9 solved Summative Assesment

           CBSE Class IX ( 9th) Science
        Chapter 4. Structure Of The Atom

Multiple Choice Questions ( 1marks)                1 x  5  =  5 marks
1. Which of the following correctly represent the electronic distribution in the Mg atom?
(a) 3, 8, 1          (b) 2, 8, 2         (c) 1, 8, 3        (d) 8, 2, 2
2. Rutherford’s ‘alpha (α) particles scattering experiment’ resulted in to discovery of
(a) Electron   (b) Proton      (c) Nucleus in the atom       (d) Atomic mass
3. The ion of an element has 3 positive charges. Mass number of the atom is 27 and the number of neutrons is 14. What is the number of electrons in the ion?
(a) 13             (b) 10             (c) 14                          (d) 16
4. Elements with valency 1 are
(a) Always metals    (b) always metalloids          (c) either metals or non-metals      (d) always non-metals
5. The first model of an atom was given by
(a) N. Bohr                (b) E. Goldstein                    (c) Rutherford                       (d) J.J. Thomson

Ans: 1. (b)     2. (c)              3. (b)     4. (c)     5. (d)

Very Short Answer Questions                 2 x  5  =  10 marks

6. Is it possible for the atom of an element to have one electron, one proton and no neutron? If so, name the element.

Ans: Yes, it is true for hydrogen atom which is represented as 1H1

7. Why did Rutherford select a gold foil in his α–ray scattering experiment?

Ans it is because gold has high malleability can be hammered into thin sheet
8. Will Cl-35  and Cl-37 have different valences?

Ans: No, It is because these are isotopes of chlorine that have same atomic number but different mass number

9. Calculate the number of neutrons present in the nucleus of an element X which is represented as 31 X15 .

Ans: 31 X15 .indicate that No. of proton=15 and mass number =31
         Mass number = No. of protons + No. of neutrons = 31
         Number of neutrons = 31– number of protons = 31–15 = 16

10. The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both these elements is 40. What is the name given to such a pair of elements?

Ans: Isobars

Short Answer Questions (3 marks)        3 x 5  =  15 marks

11. Why do Helium, Neon and Argon have a zero valency?

Ans: Helium, Neon and Argon have 2, 8 and 8 electron in outermost cell so they are having no need to gain or loss electrons .Hence they have zero valency.

12. In what way the Rutherford  proposed atomic model?

Ans: Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits. There is a positively charged centre 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.

13. In what way the Thomson proposed atomic model?

Ans: 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.

14. What were the drawbacks of Rutherford’s model of an atom?

Ans: The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo 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.

15. What are the limitations of J.J. Thomson’s model of an atom?

Ans. The major limitation of J.J. Thomson’s model is that it does not explain how positively charged particles are shielded from negatively charged particles, without getting neutralized.

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