NCERT Exemplar Solutions for Class 9 Science Chapter 4 Structure of the Atom (MCQ, SAQ and LAQ)
Chapter Name | NCERT Exemplar Solutions for Class 9 Science Ch 4 Structure of the Atom |
Topics Covered |
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Related Study |
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Objective Type Questions for Structure of the Atom
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
Solution
(b) 2, 8, 2
2. Rutherford’s ‘alpha (a ) particles scattering experiment’ resulted in to discovery of :
(a) Electron
(b) Proton
(c) Nucleus in the atom
(d) Atomic mass
Solution
(c) Nucleus in the atom
3. The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?
(a) 15X31
(b) 16X31
(c) 15X16
(d) 16X15
Solution
(a) 15X31
4. Dalton’s atomic theory successfully explained :
(i) Law of conservation of mass
(ii) Law of constant composition
(iii) Law of radioactivity
(iv) Law of multiple proportions
(a) (i), (ii) and (iii)
(b) (i), (iii) and (iv)
(c) (ii), (iii) and (iv)
(d) (i), (ii) and (iv)
Solution
(d) (i), (ii) and (iv)
5. Which of the following statements about Rutherford’s model of atom are correct?
(i) considered the nucleus as positively charged
(ii) established that the a-particles are four times heavy as a hydrogen atom
(iii) can be compared to solar system
(iv) was in agreement with Thomson’s model
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) Only (i)
Solution
(a) (i) and (iii)
6. Which of the following are true for an element?
(i) Atomic number = number of protons + number of electrons
(ii) Mass number = number of protons + number of neutrons
(iii) Atomic mass = number of protons = number of neutrons
(iv) Atomic number = number of protons = number of electrons
(a) (i) and (ii)
(b) (i) and (iii)
(c) and (iii)
(d) (ii) and (iv)
Solution
(d) (ii) and (iv)
7. In the Thomson’s model of atom, which of the following statements are correct?
(i) The mass of the atom is assumed to be uniformly distributed over the atom.
(ii) The positive charge is assumed to be uniformly distributed over the atom.
(iii) The electrons are uniformly distributed in the positively charged sphere.
(iv) The electrons attract each other to stabilise the atom.
(a) (i), (ii) and (iii)
(b) (i) and (iii)
(c) (i) and (iv)
(d) (i), (iii) and (iv)
Solution
(a) (i), (ii) and (iii)
8. Rutherford’s a -particle scattering experiment showed that :
(i) Electrons have negative charge.
(ii) The mass and positive charge of the atom is concentrated in the nucleus.
(iii) Neutron exists in the nucleus.
(iv) Most of the space in atom is empty. Which of the above statements are correct?
(a) (i) and (iii)
(b) (ii) and (iv)
(c) (i) and (iv)
(d) (iii) and (iv)
Solution
(b) (ii) and (iv)
9. 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
Solution
(b) 10
10. Identify the Mg2+ ion from the fig. where, n and p represent the number of neutrons and protons respectively :
Solution
11. In a sample of ethyl ethanoate (CH3COOC2H5), the two oxygen atoms have the same number of electrons but different number of neutrons. Which of the following is the correct reason for it?
(a) One of the oxygen atoms has gained electrons.
(b) One of the oxygen atoms has gained two neutrons.
(c) The two oxygen atoms are isotopes.
(d) The two oxygen atoms are isobars.
Solution
(c) The two oxygen atoms are isotopes.
12. Elements with valency 1 are :
(a) Always metals
(b) Always metalloids
(c) Either metals or non-metals
(d) Always non-metals
Solution
(c) Either metals or non-metals
13. The first model of an atom was given by :
(a) N. Bohr
(b) E. Goldstein
(c) Rutherford
(d) J.J. Thomson
Solution
(d) J.J. Thomson
14. An atom with 3 protons and 4 neutrons will have a valency of :
(a) 3
(b) 7
(c) 1
(d) 4
Solution
(c) 1
15. The electron distribution in an aluminium atom is :
(a) 2, 8, 3
(b) 2, 8, 2
(c) 8, 2, 3
(d) 2, 3, 8
Solution
(a) 2, 8, 3
16. Which of the following fig. do not represent Bohr’s model of an atom correctly?
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (ii) and (iv)
(d) (i) and (iv)
Solution
(c) (ii) and (iv)
17. Which of the following statement is always correct?
(a) An atom has equal number of electrons and protons.
(b) An atom has equal number of electrons and neutrons.
(c) An atom has equal number of protons and neutrons.
(d) An atom has equal number of electrons, protons and neutrons.
Solution
(a) An atom has equal number of electrons and protons.
18. Atomic models have been improved over the years. Arrange the following atomic models in the order of their chronological order :
(i) Rutherford’s atomic model,
(ii) Thomson’s atomic model,
(iii) Bohr’s atomic model.
(a) (i), (ii) and (iii)
(b) (ii), (iii) and (i)
(c) (ii), (i) and (iii)
(d) (iii), (ii) and (i)
Solution
(c) (ii), (i) and (iii)
Short Answer Questions Structure of the Atom
19. Is it possible for the atom of an element to have one electron, one proton and no neutron? If so, name the element.
Solution
1 proton means atomic no. (Z) = 1
1 neutron means mass no. (A) = p + n = 1 + 1 = 2
1 electron and 1 proton mean that atom is electrically neutral.
Hence, the element is 11H (An isotop of Hydrogen – Deuterium)
20. Write any two observations which support the fact that atoms are divisible.
Solution
Atoms are made up of electrons, protons and neutrons. Atoms can undergo nuclear fission, i.e. atoms can be split into smaller atoms by hitting nuclei of heavy atoms with subatomic particles.
21. Will 35Cl and 37Cl have different valencies? Justify your answer.
Solution
No, 35Cl and 37Cl both atoms have same valency because these isotopes have same atomic number : 17. So, their electronic configuration is same : 2, 8, 7.
Hence, both atoms have valency 1.
22. Why did Rutherford select a gold foil in his a-ray scattering experiment?
Solution
Rutherford selected a gold foil in his a -ray scattering experiment because gold is a heavy metal with high atomic number and it is highly malleable too. While any light metal cannot be used as by hitting through alpha particles, the atom of light metal will be pushed forward and scattering would not be occur.
23. Find out the valency of the atoms represented by the fig. (a) and (b).
Solution
(a) 0, (b) –1
24. One electron is present in the outermost shell of the atom of an element X. What would be the nature and value of charge on the ion formed if this electron is removed from the outermost shell?
Solution
+1.
25. Write down the electron distribution of chlorine atom. How many electrons are there in the L shell? (Atomic number of chlorine is 17.)
Solution
17Cl = K-2, L-8, M-7
Thus, L shell has 8 electrons.
26. In the atom of an element X, 6 electrons are present in the outermost shell. If it acquires noble gas configuration by accepting requisite number of electrons, then what would be the charge on the ion so formed?
Solution
Charge of an ion = –2, because atom will gain 2 electrons to get noble gas configuration.
27. What information do you get from the fig. about the atomic number, mass number and valency of atoms X, Y and Z? Give your answer in a tabular form.
Solution
|
Sl. No. |
Atomic number |
Mass number |
Valency |
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X |
5 |
11 |
3 |
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Y |
8 |
18 |
2 |
|
Z |
15 |
31 |
3, 5 |
28. In response to a question, a student stated that in an atom, the number of protons is greater than the number of neutrons, which in turn is greater than the number of electrons. Do you agree with the statement? Justify your answer.
Solution
This statement is not right. As number of electron = number of proton in neutral atom. Number of neutron can be greater than the number of electron. Number of neutron can be equal to or greater than the number of protons because mass number is equal to double the atomic number.
29. Calculate the number of neutrons present in the nucleus of an element X which is represented as 1531X.
Solution
Atomic number = 15
Mass number = 31
Number of electron = A – Z = 31 – 15 = 16
30. Match the names of the scientists given in column A with their contributions towards the understanding of the atomic structure as given in column B :
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Column A |
Column B |
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(a) |
Ernest Rutherford |
(i) |
Indivisibility of atoms |
|
(b) |
J.J. Thomson |
(ii) |
Stationary orbits |
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(c) |
Dalton |
(iii) |
Concept of nucleus |
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(d) |
Neils bohr |
(iv) |
Discovery of electrons |
|
(e) |
James Chadwick |
(v) |
Atomic number |
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(f) |
E. Goldstein |
(vi) |
Neutron |
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(g) |
Mosley |
(vii) |
Canal rays |
Solution
|
(a) |
Ernest Rutherford |
Concept of nucleus |
|
(b) |
J.J. Thomson |
Discovery of electrons |
|
(c) |
Dalton |
Indivisibility of atoms |
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(d) |
Neils Bohr |
Stationary orbits |
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(e) |
James Chadwick |
Neutron |
|
(f) |
E. Goldstein |
Canal rays |
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(g) |
Mosley |
Atomic number |
31. 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?
Solution
Atoms of different elements having different atomic numbers but same mass number are called isobars.
32. Complete the table on the basis of information available in the symbols given below :
(a) 17Cl35 (b) 6C12 (c) 35Br85
Solution
|
Elements |
No. of protons |
No. of neutrons |
|
17Cl35 |
17 |
35 – 17 = 18 |
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6C12 |
6 |
12 – 6 = 6 |
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35Br85 |
35 |
81 – 35 = 46 |
33. Helium atom has 2 electrons in its valence shell but its valency is not 2. Explain.
Solution
Helium atom has only two atoms in K-shell. K-shell is already complete. It can neither lose nor gain electron. So, its valency is zero.
34. Fill in the blanks in the following statements :
(a) Rutherford’s a -particle scattering experiment led to the discovery of the _____.
(b) Isotopes have same _____ but different _____.
(c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be _____ and _____ respectively.
(d) The electronic configuration of silicon is _____ and that of sulphur is _____.
Solution
(a) nucleus
(b) same atomic numbers, mass numbers
(c) 0 and –1
(d) Si = 2,8,4 and S = 2,8,6
35. An element X has a mass number 4 and atomic number 2. Write the valency of this element?
Solution
Atomic number = 2,
So number of electrons = 2
Electron has only K-shell containing 2 electrons.
Hence, its valency = 0. Thus, element X is 24He (noble gas).
36. Why do helium, neon and argon have a zero valency?
Solution
Helium, neon and argon have completely filled outermost shells. Thus, they have stable electronic configuration. They neither lose electrons nor gain electrons. Thus, their valency is zero.
37. Enlist the conclusions drawn by Rutherford from his a-ray scattering experiment.
Solution
Rutherford concluded from the a-particle scattering experiment that :
- Most of the space inside the atom is empty because most of the a-particles passed through the gold foil without getting deflected.
- Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
- A very small fraction of a-particles were deflected, indicating that all the positive charges and mass of the gold atom were concentrated in a very small volume within the atom. From the data he also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.
38. In what way is the Rutherford’s atomic model different from that of Thomson’s atomic model?
Solution
According to the Thomson’s atomic model, charge and mass are uniformly distributed all over the atom, whereas in Rutherford’s model of atom, the positive charge and mass of the atom are concentrated in the small nucleus in the centre of atom and negatively charged electron are distributed around the nucleus.
39. What were the drawbacks of Rutherford’s model of an atom?
Solution
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.
40. What are the postulates of Bohr’s model of an atom?
Solution
- An atom consists of heavy positively charged nucleus.
- The electrons in an atom revolve around the nucleus in definite circular paths called orbits or shells.
- Each energy level is associated with definite amount of energy and change in energy takes place when electron jumps from one energy level to another energy level.
41. Show diagrammatically the electron distributions in a sodium atom and a sodium ion and also give their atomic number.
Solution
