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CBSE Class 9 Science Question 19 of 19

Structure of the Atom — Question 2

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Question 2

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

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.

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Science | Chapter 4: Structure of the AtomWeb Content — Quick Reference

Chapter 4: Structure of the Atom — 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 — they are frequently asked in exams.
  • Focus on comparison tables, diagrams, and definitions — 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
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