Class 9th Science Structure of the Atom Practice Questions

Section A – Multiple Choice Questions (MCQs) [Q.1–20]

1. Who discovered the electron?
   a) Rutherford
   b) Thomson
   c) J. J. Thomson
   d) Chadwick
2. Proton was discovered by:
   a) Goldstein
   b) Thomson
   c) Rutherford
   d) Bohr
3. Neutron was discovered by:
   a) Goldstein
   b) Bohr
   c) Chadwick
   d) Dalton
4. Who proposed the plum pudding model of the atom?
   a) Rutherford
   b) J. J. Thomson
   c) Bohr
   d) Chadwick
5. Which experiment led to the discovery of the nucleus?
   a) Oil drop experiment
   b) Gold foil experiment
   c) Canal ray experiment
   d) Cathode ray experiment
6. The mass of an electron is approximately:
   a) 1 u
   b) 0 u
   c) 1/1836 of a proton
   d) 1/1000 of a proton
7. Which particle is negatively charged?
   a) Proton
   b) Neutron
   c) Electron
   d) Nucleus
8. Rutherford’s model of atom failed because:
   a) Electrons are stationary
   b) Electrons revolving around nucleus should lose energy
   c) Neutrons were not discovered
   d) Atom is indivisible
9. Bohr’s model explained:
   a) Stability of atom
   b) Isotopes
   c) Isobars
   d) Neutrons
10. Maximum number of electrons in L-shell is:
    a) 2
    b) 8
    c) 18
    d) 32
11. How many electron are there in hydrogen atom?
    a)1 b) 2 c) 0 d) -1
12. Atomic number is equal to:
    a) Mass number
    b) Number of neutrons
    c) Number of protons
    d) Number of nucleons
13. Mass number is:
    a) Number of protons
    b) Number of neutrons
    c) Sum of protons and neutrons
    d) Number of electrons
14. Which is an isotope of hydrogen?
    a) ²H
    b) ³He
    c) ¹²C
    d) ¹⁴N
15. Isotopes differ in:
    a) Number of protons
    b) Number of neutrons
    c) Atomic number
    d) Chemical properties
16. The number of neutrons in ¹⁴₆C is:
    a) 6
    b) 8
    c) 14
    d) 12
17. The number of electrons in ¹⁹₉F is:
    a) 9
    b) 10
    c) 19
    d) 18
18. Which is an isobar pair?
    a) ¹⁴₆C and ¹⁴₇N
    b) ¹⁴₆C and ¹²₆C
    c) ¹H and ²H
    d) ¹⁶₈O and ¹⁷₈O
19. The electronic configuration of sodium is:
    a) 2, 8, 2
    b) 2, 8, 1
    c) 2, 6, 1
    d) 2, 7, 2
20. Maximum number of electrons in M shell:
    a) 8
    b) 18
    c) 32
    d) 2

Section B – True/False [Q.21–30]

21. Atom is indivisible according to Dalton.
22. Proton is positively charged.
23. Electron is heavier than proton.
24. Neutron has no charge.
25. Atomic number is equal to number of protons.
26. Mass number is equal to number of protons only.
27. Isotopes have same mass number but different atomic number.
28. Isobars have same atomic number but different mass numbers.
29. Bohr’s model explains distribution of electrons in shells.
30. K shell can hold a maximum of 8 electrons.

Section C – Fill in the Blanks [Q.31–40]

31. The subatomic particle discovered by J. J. Thomson is __________.
32. Neutrons are located in the __________.
33. The symbol of tritium is __________.
34. The maximum number of electrons in a shell is given by the formula __________.
35. ¹H, ²H, ³H are __________ of hydrogen.
36. ⁴⁰Ca and ⁴⁰Ar are __________.
37. Rutherford’s experiment used __________ foil.
38. __________ discovered canal rays.
39. The central positively charged part of an atom is called __________.
40. An atom with 17 protons will have __________ electrons.

Section D – Assertion & Reason [Q.41–50]

41. Assertion: Atoms are indivisible.
    Reason: Dalton’s theory considered atoms indivisible.
42. Assertion: Nucleus is positively charged.
    Reason: Protons are positively charged and present in nucleus.
43. Assertion: Electrons revolve in fixed orbits.
    Reason: Bohr explained stability of atoms.
44. Assertion: Isotopes have different number of neutrons.
    Reason: Isotopes have same atomic number but different mass numbers.
45. Assertion: Atomic number determines the identity of an element.
    Reason: Atomic number = Number of protons.
46. Assertion: Rutherford’s model explained stability.
    Reason: Moving electrons should continuously lose energy.
47. Assertion: Isobars have same number of neutrons.
    Reason: Isobars have same mass number.
48. Assertion: Neutrons carry positive charge.
    Reason: Neutrons are located in nucleus.
49. Assertion: M shell can accommodate maximum 8 electrons.
    Reason: Formula 2n² decides capacity of a shell.
50. Assertion: Isotopes of an element have identical chemical properties.
    Reason: They have the same number of protons and electrons.

Section E – Very Short Answer (1 Mark Each) [Q.51–60]

51. Name the subatomic particles of an atom.
52. Who discovered neutron?
53. Write the electronic configuration of oxygen.
54. Define atomic number.
55. Define mass number.
56. Which isotope of hydrogen is radioactive?
57. Write the electronic configuration of chlorine.
58. What is the maximum number of electrons in N shell?
59. Which element has atomic number 20?
60. What is the charge on neutron?

Section F – Short Answer (2 Marks Each) [Q.61–70]

61. Draw and explain Bohr’s model of sodium atom. (Fig: 2,8,1 distribution)
62. State two drawbacks of Rutherford’s model of atom.
63. Define isotopes with one example.
64. Define isobars with one example.
65. How many protons, electrons, and neutrons are present in ³⁷₁₇Cl?
66. Write the electronic configuration of Mg and Na.
67. Why are isotopes chemically similar?
68. Define valency with an example.
69. Calculate number of protons, neutrons in ¹⁴₆C.
70. Draw Bohr’s model of lithium atom. (Fig: 2,1 distribution)

Section G – Short Answer (3 Marks Each) [Q.71–80]

71. Describe Rutherford’s gold foil experiment with diagram. (Fig: alpha rays scattering)
72. State main features of Rutherford’s nuclear model.
73. Explain Bohr’s model of atom with diagram. (Shell representation)
74. Compare isotopes and isobars with examples.
75. Write electronic configuration of first 20 elements.
76. Calculate number of neutrons in: ¹⁶₈O, ³²₁₆S, ³⁵₁₇Cl.
77. Explain why isotopes of hydrogen differ in mass.
78. Draw structure of helium atom (Bohr’s model).
79. Why is atom electrically neutral?
80. Compare Dalton, Thomson, Rutherford, and Bohr models of atom.

Section H – Long Answer (4–5 Marks Each) [Q.81–90]

81. Describe J. J. Thomson’s model of atom with diagram. (Plum pudding model)
82. Explain Rutherford’s gold foil experiment in detail with diagram.
83. State postulates of Bohr’s model of atom with diagram.
84. Explain electronic configuration using 2n² rule.
85. Compare isotopes, isobars, and isotones with examples.
86. Calculate protons, neutrons, and electrons in: ²³Na, ³⁹K, ⁴⁰Ca.
87. Discuss applications of isotopes in medicine and industry.
88. Explain stability of atoms based on Bohr’s model.
89. Describe distribution of electrons in shells of first 20 elements with diagram.
90. Explain how atomic number and mass number are related to composition of an atom.

Section I – Case Study Based [Q.91–100]

Case Study 1 (Q.91–95):
In Rutherford’s gold foil experiment, alpha particles were bombarded on thin gold foil. Most passed through, some were deflected, and very few bounced back.

91. What did the deflection of alpha particles prove?
92. What does it indicate about the structure of an atom?
93. Draw Rutherford’s atomic model.
94. Why did most alpha particles pass through undeflected?
95. State one limitation of Rutherford’s model.

Case Study 2 (Q.96–100):
The element chlorine exists as two isotopes: ³⁵₁₇Cl (75%) and ³⁷₁₇Cl (25%).

96. Calculate average atomic mass of chlorine.
97. Why are these isotopes chemically identical?
98. How many protons, neutrons, and electrons are there in ³⁷₁₇Cl?
99. Draw Bohr’s model of chlorine atom (2,8,7).
100. State one application of chlorine isotopes.