Important Questions Class 9 Science Chapter 4: Chapter 4 of Class 9 Science, "Structure of the Atom," focuses on the discovery and development of atomic models. It begins with Dalton's atomic theory, followed by J.J.
Thomson’s plum pudding model and Rutherford’s nuclear model, which proposed that atoms consist of a small, dense nucleus surrounded by electrons.
This chapter introduces Bohr’s model, where electrons revolve in specific orbits, and discusses isotopes, isobars, and the composition of atoms, including protons, neutrons, and electrons.
Key concepts include atomic number, mass number, and electronic configuration, which help explain the arrangement of electrons in an atom's different energy levels.
This makes the structure of the atom important question a crucial part of study in this chapter on class 9 science chapter 4 important questions.
Many of the chapter 4 science class 9 Structure of the Atom important questions cover these topics, helping students grasp fundamental atomic concepts. For exam preparation, referring to important questions for class 9 science chapter 4 ensures a focused revision on essential points.
Important Questions Class 9 Science Chapter 4 Overview
NCERT Chapter 4 Structure of the Atom in Class 9 Science is crucial for building a strong foundation in atomic theory. Important questions often focus on topics like the discovery of subatomic particles (electrons, protons, neutrons), atomic models (Dalton’s, Thomson’s, Rutherford’s, and Bohr’s), and differences between isotopes and isobars.
Questions related to atomic number, mass number, and electronic configuration are essential as they frequently appear in exams and form the basis for higher-level chemistry.
Understanding these topics helps students grasp complex concepts in future classes, making the questions from this chapter vital for academic success and competitive exams.
Structure of the Atom Important Question Important Questions Class 9 Science Chapter 4
Below is the Important Questions Class 9 Science Chapter 4 Structure of the Atom -
1.The number of valence electrons in Cl⁻ ion are:
(A) 16
(B) 8
(C) 17
(D) 18
Answer: (B) 8
2. Isotopes of an element have
(A) The same physical properties
(B) Different chemical properties
(C) Different numbers of neutrons
(D) Different atomic numbers
Answer: (C) Different numbers of neutrons
3. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(A) Atomic Nucleus
(B) Electron
(C) Proton
(D) Neutron
Answer: (A) Atomic Nucleus
4. Atomic Number of a neutral atom is equal to:
(A) Number of Protons
(B) Number of Electrons
(C) Number of Neutrons
(D) Both (A) and (B)
Answer: (D) Both (A) and (B)
5. The charge of proton (p⁺) is:
(A) +1.6 × 10⁻¹⁹ C
(B) –1.6 × 10⁻¹⁹ C
(C) +1.6 × 10¹⁹ C
(D) –1.6 × 10¹⁹ C
Answer: (A) +1.6 × 10⁻¹⁹ C
6. ²¹₁₀Ne and ²²₁₀Ne are
(A) Isotopes
(B) Isobars
(C) Isotones
(D) Both (A) and (B)
Answer: 
7. ⁴₂He has:
(A) 2p⁺ and 2n⁰
(B) 2p⁺ and 4n⁰
(C) 4p⁺ and 2n⁰
(D) 2p⁺ and 4n⁰
Answer: (A) 2p⁺ and 2n⁰
8. Atomicity of fluorine is:
(A) 1
(B) 2
(C) 3
(D) 4
Answer: (B) 2
9. What will be the charge on the atom containing 7 protons and 9 electrons?
(A) –1
(B) +1
(C) –2
(D) +2
Answer: (C) –2
10. Which of the following statements are correct?
(i) Na⁺, O²⁻ and F⁻ are isoelectronic.
(ii) All isotopes of hydrogen are isotonic.
(iii) ⁴⁰Ar and ⁴⁰Ca are called isobars.
(iv) Co-60 isotope is used in the treatment of cancer.
(A) (i), (ii), (iii)
(B) (i), (iii), (iv)
(C) (ii), (iii), (iv)
(D) (i), (ii), (iii), (iv)
Answer: (B) (i), (iii), (iv)
11. An ion X²⁻ contains 10 electrons and 8 neutrons. The atomic number and mass number of the element are:
(A) 10, 18
(B) 8, 16
(C) 10, 16
(D) 8, 18
Answer: (B) 8, 16
12. An ion M³⁺ contains 10 electrons and 14 neutrons. The atomic number and mass number of the element are:
(A) 13, 24
(B) 13, 27
(C) 7, 21
(D) 10, 24
Answer: (B) 13, 27
13. In Thomson’s model of the atom, which of the following statements is correct?
(i) The mass of the atom is assumed to be uniformly distributed over the atom.
(ii) The 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 stabilize the atom.
(A) (i), (ii), and (iii)
(B) (i) and (iii)
(C) (i) and (iv)
(D) (i), (iii), and (iv)
Answer: (A) (i), (ii), and (iii)
14. What is the number of electrons, protons, and neutrons in a magnesium atom? Given that the atomic number of magnesium is 12 and its mass number is 24.
(A) 12, 12, 24
(B) 12, 24, 12
(C) 24, 12, 12
(D) 12, 12, 12
Answer: (D) 12, 12, 12
15. Which of the following statements about the Rutherford model of the atom are correct?
(i) Considered the nucleus as positively charged.
(ii) Established that the alpha particles are as heavy as hydrogen atom.
(iii) Can be compared to the solar system.
(iv) Agreed with Thomson’s model.
(A) (i) and (iii)
(B) (ii) and (iii)
(C) (i) and (iv)
(D) Only (i)
Answer: (A) (i) and (iii)
16. If an atom contains one electron and one proton, will it carry any charge or not?
Answer: It will be neutral.
17. Name the three sub-atomic particles of an atom.
Answer: The three subatomic particles of an atom are Proton, Neutron, and Electron.
18. Based on Thomson’s model of an atom, explain how the atom is neutral as a whole.
Answer: As per Thomson’s model of an atom, the number of electrons and the number of protons are equal in an atom. Electrons are negatively charged and protons are positively charged; hence the positive and negative charges are neutralized by each other which makes atoms neutral as a whole.
19. Draw a sketch of Bohr’s model of an atom with three shells.
Answer: Bohr’s model of an atom with three shells is as follows:
20. What do you think would be the observation if the α particle scattering experiment is carried out using a foil of a metal other than gold?
Answer: If the α particle scattering experiment is carried out using a foil of a metal other than gold, then there would be no change in the observation because the basic structure of an atom remains the same.
21. What was the reason for selecting a gold foil by Rutherford in his α-ray scattering experiment?
Answer: Rutherford selected a gold foil in his α-ray scattering experiment because gold is highly malleable and can be hammered into thin sheets.
22. Calcium and argon have atomic numbers 20 and 18, respectively, but both of them have a mass number of 40. What is the name used for the pair of elements like this?
Answer: ‘Isobars’ is the term used for the pair of elements having the same mass number but different atomic numbers.
23. Helium atom has an atomic mass of 4u and two protons in its nucleus. How many neutrons does it have?
Answer: The atomic mass of an atom is the sum of masses of protons and neutrons present in its nucleus.
Given that the mass of the helium atom is 4 u and two protons are present in its nucleus.
So the number of neutrons will be
Number of neutrons
= Atomic mass – Number of protons
⇒ Number of neutrons = 4 – 2
∴ Number of neutrons = 2
Therefore, the helium atom has 2 neutrons.
24. If the K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer: The K shell contains 2 electrons and the L shell contains 8 electrons. If the K and L shells of an atom are full, then the total number of electrons in the atom will be 10.
25. If number of electrons in an atom is 8 and number of protons is also 8, then
i. What is the atomic number of the atom?
Sol. The atomic number of an atom is equal to the number of protons present in its nucleus. So the atomic number of an atom with 8 protons is 8.
ii. What is the charge on the atom?
Sol. A single electron contains one negative charge and one single proton contains one positive charge. There are equal numbers of electrons and protons in an atom so they will neutralize each other. The atom will be neutral.
26. What are the limitations of J.J. Thomson’s model of the atom?
Sol. Limitations of J.J Thomson's model of an atom: ∙ Thomson’s model failed to explain the results of the alpha particle scattering experiment that was given by Rutherford.
- One of the major drawbacks of his model is that there were no experimental pieces of evidence provided by him.
- He failed to explain the stability of atoms.
27. If Z = 3, what would be the valency of the element? Also, name the element.
Sol. Z = 3 represents that element has 3 electrons in its shells. The electronic configuration is 2, 1. It means the outermost shell electron has 1 electron, so its valency is 1. The element is Lithium.
28. Write the electronic configuration of following entities.
(a) Cl–
(b) Mg
(c) Al3+
(d) O
Sol. a) Cl– ⇒ Electronic Configuration = 2, 8, 8 b) Mg ⇒ Electronic Configuration = 2, 8, 2 c) Al3+ ⇒ Electronic Configuration = 2, 8 d) O ⇒ Electronic Configuration = 2, 6.
29. (a) An ion with the symbol X2+ has 10 electrons and 12 neutrons. Find the atomic number and mass number of the element X.
(b) Can a neutral atom of an element contain 12 electrons and 13 protons?
Ans (a) X2+ ion contains 10 electrons. Therefore, number of electrons in element X is 12.
Number of electrons = Number of protons
Atomic number = Number of protons
Hence, the Atomic number of element X is 12,
Mass number = Number of protons + Number of neutrons
= 12 + 12
= 24
Hence, the mass number of element X = 24.
(b) No, a neutral atom can not have 12 electrons and 13 protons. For a neutral atom the number of protons is always equal to the number of electrons. So, an atom with 12 electrons and 13 protons would be positively charged.
30. If the number of electrons in an atom is 8 and number of protons is also 8, then
(i) What is the atomic number of the atom?
(ii) What is the charge on the atom?
Sol. It is given that, number of electrons = 8 and, number of protons = 8
(i) The number of protons present in an atom is equal to the atomic number of an atom. Therefore, its atomic number is 8.
(ii) The number of protons is equal to the number of electrons in an atom. Therefore, both positive charges and negative charges will neutralize each other. Hence, the atom does not contain any charge.
(a) How many electrons are present in X and Y ?
(b) How many nucleons are present in X and Y ?
(c) Explain the term used to represent X and Y
Sol. (a) Atomic number of X = 20
Atomic number = Number of protons = 20
Number of protons = Number of electrons
Therefore, number of electrons = 20
Atomic number of Y = 18
Atomic number = Number of protons = 18
Number of protons = Number of electrons
Therefore, number of electrons = 18
Hence, the number of electrons present in element X and Y are 20 and 18 respectively.
(b) Atomic mass of element X and Y = 40 Number of Nucleons =
Number of protons + Number of neutrons = Mass number.
Therefore, X and Y contains 40 nucleons.
(c) Isobar is the term used for the pair of elements having same mass number but different atomic number.
32. If bromine atom is available in the form of two isotopes 7935Br(49.7%)and 8135Br(50.3%) , calculate the average atomic mass of bromine atom.
Sol. Atomic masses of two isotopic atoms of Br are 79(49.7 %) and 81(50.3 %) respectively.
Hence, the average atomic mass of Bromine is: 79 49.7 81 50.3
× ×
= +
100 100
= 39.263 + 40.743
= 80.006 u
33. Explain the atomic model proposed by Rutherford.
Sol. Rutherford’s Atomic Model:
(i) He proposed that an atom consists of a tiny positively charged nucleus at its center and the entire mass of an atom is concentrated in the nucleus.
(ii) The size of the nucleus is very small compared to the size of an atom.
(iii) He also stated that the electrons revolve around the nucleus in a circular path.
(iv) He said that electrons and nucleus are held together by a strong electrostatic force of attraction.
34. An atom of an element contains three electrons in the third shell (outermost shell). Write:
(a) The electronic configuration of the element
(b) Number of protons present in the element
(c) The atomic number of the element
(d) Valency of that element
(e) The name of the element
Sol. M-shell is the third shell. If an atom contains three electrons in its third shell, i.e., M shell that means, its K and L shells are completely filled.
(a) Electronic configuration of element: K, L, M
2, 8, 3
(b) Total number of electrons = 13
For a neutral atom,
Number of protons = Number of electrons
Therefore, number of protons = 13
(c) Atomic number of element = Number of protons Therefore, atomic number of element = 13
(d) Electronic configuration of element: K, L, M
2, 8, 3
Therefore, the valency of the element = 3
(e) The element has atomic number 13, therefore, element is Aluminum (Al).
35. Differentiate between Rutherford’s atomic model and J.J. Thomson’s atomic model.
Benefits of Solving Important Questions Class 9 Science Chapter 4
Solving important questions from Chapter 4, "Structure of the Atom," in Class 9 Science offers several benefits:
Concept Clarity : It helps in thoroughly understanding key concepts like atomic models, subatomic particles, and electronic configuration, which are fundamental for higher-level chemistry.
Exam Preparation : Important Questions Class 9 Science Chapter 4 are often designed based on exam patterns, helping students become familiar with commonly asked topics, improving their confidence for exams.
Problem-Solving Skills : Practicing a variety of questions enhances analytical thinking and problem-solving abilities, preparing students for complex topics in future studies.
Foundation Building : Mastery of atomic structure forms the foundation for advanced topics in chemistry and physics, aiding academic progression.
Better Retention : Regular practice ensures better retention of key facts, formulas, and definitions, making revision easier.
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