Question 1

Question 5Compare all the proposed models of an atom given in this chapter.

Accepted Answer

Thomson's ModelRutherford's ModelBohr's modelIt was proposed by J.J. Thomson.It was proposed by Ernst Rutherford.It was proposed by Neil Bohr.An atom consists of a positively charged sphere and the electrons are embedded in it.There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.There is very large empty space between nucleus and electrons.While revolving in discrete orbits the electrons do not radiate energy.It could not explain the result of Rutherford's ∝–particle scattering experiment.It could not explain the stability of atom because revolving electron will lose energy and will ultimately fall into the nucleus.This model has overcome the limitations of the Rutherford model by defining energy levels for electrons.

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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 2

Question 6Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Accepted Answer

The following rules are followed for writing the number of electrons in different energy levels or shells for the first eighteen elements:The maximum number of electrons in each shell or orbit is determined by the formula 2n2, where n is the shell number.K shell, n=1, no. of electrons = 2 x 12 = 2L shell, n=2, no. of electrons = 2 x 22 = 8M shell, n=3, no. of electrons = 2 x 32 = 18Electrons are not accommodated in a given shell, unless the inner shells are filled. Shells are filled in step-wise manner.The atom's outermost shell needs 2 electrons for stability (duplet rule) if it has only one shell. If it has multiple shells, it requires 8 electrons (octet rule) in the outermost shell for stability.

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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 3

Question 7Define valency by taking examples of silicon and oxygen.

Accepted Answer

The definite combining capacity of the atoms of each element, wherein electrons are lost, gained or shared to make the octet of electrons present in the outermost shell, is defined as valency.(i) The electronic configuration of silicon [14] is 2, 8, 4. It has 4 electrons in its outermost shell. So, the valency of silicon = 8 – the number of electrons in the outermost shell = 8 - 4 = 4.Hence, valency of silicon = 4.(ii) The electronic configuration of oxygen [8] is 2, 6. It has 6 (more than 4) electrons in its outermost shell. So, the valency of oxygen = 8 – the number of electrons in the outermost shell = 8 − 6 = 2.Hence, valency of oxygen = 2.

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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 4

Question 8Explain with examples(i) Atomic number,(ii) Mass number,(iii) Isotopes and(iv) Isobars.Give any two uses of isotopes.

Accepted Answer

(i) Atomic number — The number of protons present in the nucleus of an atom is defined as the atomic number and is denoted by Z.Example: Atomic number of hydrogen (Z) = 1 as only one proton is present in the nucleus of hydrogen atom.(ii) Mass number — The total number of protons and neutrons present in the nucleus of an atom is known as the mass number and is denoted by A.Example: Mass number of Carbon (A) = 12 as it has 6 protons and 6 neutrons.(iii) Isotopes — The atoms of the same element having the same atomic number but different mass number are known as isotopes.Example: Carbon has two isotopes 6C12 and 6C14(iv) Isobars — Atoms of different elements with different atomic numbers, which have same mass number are known as isobars.Example: Calcium [20Ca40] atomic number 20 and argon [18Ar40] atomic number 18 are isobars as both have same mass number i.e., 40 and different atomic numbers.Isotopes have special properties which makes them useful in various fields like:An isotope of uranium is used as a fuel in nuclear reactors.An isotope of cobalt is used in the treatment of cancer.

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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 5

Question 9Na+ has completely filled K and L shells. Explain.

Accepted Answer

The atomic number of sodium is 11. It has 11 electrons in its orbitals. So, its electronic configuration is K-2, L-8, M-1. The maximum electrons in K, L, M shells according to 2n2 rule are 2, 8 and 18 respectively.In case of Na+ (10), one electron from the M shell is lost, and it obtains a positive charge. The new electronic configuration is K-1 and L-8. Hence, it has completely filled shells.

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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 6

Question 10If the bromine atom is available in the form of, say, two isotopes 3579Br (49.7%) and 3581Br (50.3%), calculate the average atomic mass of bromine atom.

Accepted Answer

The atomic mass of an element is the mass of one atom of that element.Average atomic mass takes into account the isotopic abundance.Isotope of bromine with atomic mass 79 u = 49.7%Therefore, Contribution of 3579Br to atomic mass = 79×49.7100\dfrac{79 	imes 49.7}{100}10079×49.7​ = 39.263 uIsotope of bromine with atomic mass 81 u = 50.3%Contribution of 3581Br to the atomic mass of bromine = 81×50.3100\dfrac{81 	imes 50.3}{100}10081×50.3​ = 40.743uHence, the average atomic mass of the bromine atom = 39.263 + 40.743 = 80.006 u

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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 7

Question 11The average atomic mass of a sample of element X is 16.2 u. What are the percentages of isotopes 816X and 818X in the sample?

Accepted Answer

Let the percentage of 816X be a and that of 818X be 100 - a.16.2u = 16a100\dfrac{	ext{16a}}{100}10016a​ + 18(100-a)10018\dfrac{	ext{(100-a)}}{100}18100(100-a)​⇒ 1620 = 16a + 1800 – 18a⇒ 1620 = 1800 – 2a⇒ 2a = 1800 - 1620⇒ a = 1800−16202\dfrac{1800 - 1620}{2}21800−1620​⇒ a = 1802\dfrac{180}{2}2180​⇒ a = 90∴ Percentage of 816X = 90%andPercentage of 818X = 100 - a = 100 - 90 = 10%Hence, the percentage of the isotope 816X in the sample is 90% and that of 818X is 10%

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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 8

Question 12If Z=3, what would be the valency of the element? Also, name the element.

Accepted Answer

Given,Atomic number, Z = 3The electronic configuration of the element = 2,1. Hence, its valency = 1The element with atomic number 3 is Lithium.

Web Content: Structure of the Atom | Bright Tutorials

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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 9

Question 13Composition of the nuclei of two atomic species, X and Y, are given as under XYProtons =66Neutrons =68Give the mass numbers of X and Y. What is the relation between the two species?

Accepted Answer

Mass number = No. of protons + No. of neutronsMass number of X = 6 + 6 = 12Mass number of Y = 6 + 8 = 14Number of protons is same in both so atomic number is same however as the number of neutrons is different so mass number is different. Hence, these two elements are isotopes.

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    .keyword-tags span { background: var(--bt-primary-light); color: var(--bt-primary-dark); padding: 3px 10px; border-radius: 12px; font-size: 0.78rem; font-weight: 600; }

BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 10

Question 14For the following statements, write T for True and F for False.(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.(c) The mass of an electron is about 12000\dfrac{1}{2000}20001​ times that of a proton.(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

Accepted Answer

(a) False.Reason — Thomson proposed that:An atom consists of a positively charged sphere and the electrons are embedded in it.The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.(b) False.Reason — Neutron is not formed by an electron and a proton combining together. It is present in the nucleus of an atom alongside protons.(c) True.Reason — The mass of an electron is about 12000\dfrac{1}{2000}20001​ times that of a proton.(d) False.Reason — Tincture iodine is an antiseptic that usually contains 2-7% elemental I-127 dissolved in a mixture of ethanol and water. Normal iodine is used in its preparation, not its isotope. ​

Web Content: Structure of the Atom | Bright Tutorials

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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 11

Question 15Rutherford's alpha–particle scattering experiment was responsible for the discovery ofAtomic nucleusElectronProtonNeutron

Accepted Answer

Atomic nucleusReason — Rutherford's alpha–particle scattering experiment was responsible for the discovery of atomic nucleus.

Source: This question is from Structure of the Atom, Science &mdash; Class 9, CBSE Board.
Key Concepts CoveredThis question tests your understanding of the following concepts from the chapter Structure of the Atom: Question, Rutherford&#x27;s, Alpha, Particle, Scattering, Experiment. These are fundamental topics in Science that students are expected to master as part of the CBSE Class 9 curriculum.A thorough understanding of these concepts will help you answer similar questions confidently in your CBSE examinations. These topics are frequently tested in both objective and subjective sections of Science papers. We recommend revising the relevant section of your textbook alongside practising these solved examples to build a strong foundation.
How to Approach This QuestionRead the question carefully and identify what is being asked. Break down complex questions into smaller parts. Use the terminology and concepts discussed in this chapter. Structure your answer logically — begin with a definition or key statement, then provide supporting details. Review your answer to ensure it addresses all parts of the question completely.
Key Points to RememberRead the question carefully and identify all parts before answering.
Use the terminology specific to this subject and chapter.
Review the textbook content for this chapter before attempting questions.
Practice writing concise, well-structured answers within time limits.
Related Questions from Structure of the AtomQ1: Question 1 How will you find the valency of chlorine, sulphur and magnesium?
Q10: Question 10 If the bromine atom is available in the form of, say, two isotopes 35 79 Br (49.7%) and...
Q11: Question 11 The average atomic mass of a sample of element X is 16.2 u. What are the percentages of...
Q12: Question 12 If Z=3, what would be the valency of the element? Also, name the element.
Q13: Question 13 Composition of the nuclei of two atomic species, X and Y, are given as under X Y...
Practice more questions from Structure of the Atom &mdash; Science, Class 9 CBSE

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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 12

Question 16Isotopes of an element haveThe same physical propertiesDifferent chemical propertiesDifferent number of neutronsDifferent atomic numbers

Accepted Answer

Different number of neutronsReason — Isotopes of an element have same atomic number (i.e., number of protons) and different mass number (i.e., sum of protons + neutrons).

Web Content: Structure of the Atom | Bright Tutorials

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    .quick-rev h3 { color: var(--bt-primary); margin-top: 0; }
    .quick-rev ul { margin: 0; padding-left: 20px; }
    .quick-rev li { margin-bottom: 6px; font-size: 0.92rem; }
    .keyword-tags { display: flex; flex-wrap: wrap; gap: 6px; margin: 10px 0; }
    .keyword-tags span { background: var(--bt-primary-light); color: var(--bt-primary-dark); padding: 3px 10px; border-radius: 12px; font-size: 0.78rem; font-weight: 600; }

BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 13

Question 17Number of valence electrons in Cl- ion are1681718

Accepted Answer

8Reason — The electronic distribution of Cl is 2, 8, 7. There are 7 valence electrons in chlorine. It gains one electron and forms Cl-. Hence, its valency is 8.

Web Content: Structure of the Atom | Bright Tutorials

.quick-rev { background: linear-gradient(135deg, #eaf2f8, #d6eaf8); border: 2px solid var(--bt-primary-light); border-radius: 10px; padding: 20px; margin: 15px 0; }
    .quick-rev h3 { color: var(--bt-primary); margin-top: 0; }
    .quick-rev ul { margin: 0; padding-left: 20px; }
    .quick-rev li { margin-bottom: 6px; font-size: 0.92rem; }
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    .keyword-tags span { background: var(--bt-primary-light); color: var(--bt-primary-dark); padding: 3px 10px; border-radius: 12px; font-size: 0.78rem; font-weight: 600; }

BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 14

Question 18Which one of the following is a correct electronic configuration of Sodium?2, 88, 2, 12, 1, 82, 8, 1

Accepted Answer

2, 8, 1Reason — Sodium has atomic number 11 and it's electronic configuration is 2,8,1.

Source: This question is from Structure of the Atom, Science &mdash; Class 9, CBSE Board.
Key Concepts CoveredThis question tests your understanding of the following concepts from the chapter Structure of the Atom: Question, Correct, Electronic, Configuration, Sodium, Structure of the Atom. These are fundamental topics in Science that students are expected to master as part of the CBSE Class 9 curriculum.A thorough understanding of these concepts will help you answer similar questions confidently in your CBSE examinations. These topics are frequently tested in both objective and subjective sections of Science papers. We recommend revising the relevant section of your textbook alongside practising these solved examples to build a strong foundation.
How to Approach This QuestionRead the question carefully and identify what is being asked. Break down complex questions into smaller parts. Use the terminology and concepts discussed in this chapter. Structure your answer logically — begin with a definition or key statement, then provide supporting details. Review your answer to ensure it addresses all parts of the question completely.
Key Points to RememberRead the question carefully and identify all parts before answering.
Use the terminology specific to this subject and chapter.
Review the textbook content for this chapter before attempting questions.
Practice writing concise, well-structured answers within time limits.
Related Questions from Structure of the AtomQ1: Question 1 How will you find the valency of chlorine, sulphur and magnesium?
Q10: Question 10 If the bromine atom is available in the form of, say, two isotopes 35 79 Br (49.7%) and...
Q11: Question 11 The average atomic mass of a sample of element X is 16.2 u. What are the percentages of...
Q12: Question 12 If Z=3, what would be the valency of the element? Also, name the element.
Q13: Question 13 Composition of the nuclei of two atomic species, X and Y, are given as under X Y...
Practice more questions from Structure of the Atom &mdash; Science, Class 9 CBSE

Web Content: Structure of the Atom | Bright Tutorials

.quick-rev { background: linear-gradient(135deg, #eaf2f8, #d6eaf8); border: 2px solid var(--bt-primary-light); border-radius: 10px; padding: 20px; margin: 15px 0; }
    .quick-rev h3 { color: var(--bt-primary); margin-top: 0; }
    .quick-rev ul { margin: 0; padding-left: 20px; }
    .quick-rev li { margin-bottom: 6px; font-size: 0.92rem; }
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    .keyword-tags span { background: var(--bt-primary-light); color: var(--bt-primary-dark); padding: 3px 10px; border-radius: 12px; font-size: 0.78rem; font-weight: 600; }

BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 15

Question 19Complete the following table.Atomic NumberMass NumberNumber of NeutronsNumber of ProtonsNumber of ElectronsName of the Atomic Species9  10      1632      Sulphur  2412      2  1      1010

Accepted Answer

Atomic NumberMass NumberNumber of NeutronsNumber of ProtonsNumber of ElectronsName of the Atomic Species9191099Fluorine1632161616Sulphur1224121212Magnesium12111Deuterium11010Protium

Web Content: Structure of the Atom | Bright Tutorials

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    .quick-rev ul { margin: 0; padding-left: 20px; }
    .quick-rev li { margin-bottom: 6px; font-size: 0.92rem; }
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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 16

Question 3Draw a sketch of Bohr's model of an atom with three shells.

Accepted Answer

Below diagram shows Bohr's model of an atom with three shells:

Web Content: Structure of the Atom | Bright Tutorials

.quick-rev { background: linear-gradient(135deg, #eaf2f8, #d6eaf8); border: 2px solid var(--bt-primary-light); border-radius: 10px; padding: 20px; margin: 15px 0; }
    .quick-rev h3 { color: var(--bt-primary); margin-top: 0; }
    .quick-rev ul { margin: 0; padding-left: 20px; }
    .quick-rev li { margin-bottom: 6px; font-size: 0.92rem; }
    .keyword-tags { display: flex; flex-wrap: wrap; gap: 6px; margin: 10px 0; }
    .keyword-tags span { background: var(--bt-primary-light); color: var(--bt-primary-dark); padding: 3px 10px; border-radius: 12px; font-size: 0.78rem; font-weight: 600; }

BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 17

Question 4What do you think would be the observation if the ∝–particle scattering experiment is carried out using a foil of a metal other than gold?

Accepted Answer

If the ∝–particle scattering experiment is carried out using a foil of a metal (as thin as gold) other than gold, then also the observation would be same because the structure of an atom, when considered individually is same.However, other metals are not as malleable as gold so it is difficult to turn those metals into such thin sheets. If thick foil is used then it will result in the change of scattering angles which might affect the predictions of the experiment.

Web Content: Structure of the Atom | Bright Tutorials

.quick-rev { background: linear-gradient(135deg, #eaf2f8, #d6eaf8); border: 2px solid var(--bt-primary-light); border-radius: 10px; padding: 20px; margin: 15px 0; }
    .quick-rev h3 { color: var(--bt-primary); margin-top: 0; }
    .quick-rev ul { margin: 0; padding-left: 20px; }
    .quick-rev li { margin-bottom: 6px; font-size: 0.92rem; }
    .keyword-tags { display: flex; flex-wrap: wrap; gap: 6px; margin: 10px 0; }
    .keyword-tags span { background: var(--bt-primary-light); color: var(--bt-primary-dark); padding: 3px 10px; border-radius: 12px; font-size: 0.78rem; font-weight: 600; }

BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 18

Question 1How will you find the valency of chlorine, sulphur and magnesium?

Accepted Answer

(i) Valency of Chlorine :The electronic configuration of chlorine = 2, 8, 7. It has 7 (more than 4) electrons in its outermost shell. So, the valency of chlorine = 8 – the number of electrons in the outermost shell = 8 - 7 = 1.Hence, the valency of Cl is 1.(ii) Valency of Sulphur:The electronic configuration of Sulphur = 2, 8, 6. It has 6 (more than 4) electrons in its outermost shell. So, the valency of S = 8 – the number of electrons in the outermost shell = 8 - 6 = 2.Hence, the valency of S is 2.(iii) Valency of Magnesium:The electronic configuration of Magnesium = 2, 8, 2. It has 2 (less than 4) electrons in its outermost shell. So, the valency of magnesium = Number of electrons in its outermost shell = 2Hence, the valency of Mg is 2.

Web Content: Structure of the Atom | Bright Tutorials

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BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Question 19

Question 2Write the electronic configuration of any one pair of isotopes and isobars.

Accepted Answer

(a) Isotopes — Carbon has two isotopes i.e., 6C12 and 6C14. They have same electronic configuration as they have same atomic numbers = 6. Hence, their electronic configuration is 2, 4.(b) Isobars — Calcium [20Ca40] atomic number 20 and argon [18Ar40] atomic number 18 are isobars as both have same mass number i.e., 40 and different atomic numbers.Electronic configuration of 20Ca40 is 2, 8, 8, 2.Electronic configuration of 18Ar40 is 2, 8, 8.

Web Content: Structure of the Atom | Bright Tutorials

.quick-rev { background: linear-gradient(135deg, #eaf2f8, #d6eaf8); border: 2px solid var(--bt-primary-light); border-radius: 10px; padding: 20px; margin: 15px 0; }
    .quick-rev h3 { color: var(--bt-primary); margin-top: 0; }
    .quick-rev ul { margin: 0; padding-left: 20px; }
    .quick-rev li { margin-bottom: 6px; font-size: 0.92rem; }
    .keyword-tags { display: flex; flex-wrap: wrap; gap: 6px; margin: 10px 0; }
    .keyword-tags span { background: var(--bt-primary-light); color: var(--bt-primary-dark); padding: 3px 10px; border-radius: 12px; font-size: 0.78rem; font-weight: 600; }

BRIGHT TUTORIALS
  BRIGHT TUTORIALSCBSE Class IX | Academic Year 2026-20279403781999Excellence in Education
  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

BRIGHT TUTORIALSContact: 9403781999 | CBSE Class IX Coaching | 2026-2027

Structure of the Atom

Exercises

Intext Questions 2

Intext Questions 5

Intext Questions 7