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CBSE Class 9 Science Question 7 of 19

Structure of the Atom — Question 11

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Question 11

The 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?

Answer

Let the percentage of 816X be a and that of 818X be 100 - a.

16.2u = 16a100\dfrac{\text{16a}}{100} + 18(100-a)10018\dfrac{\text{(100-a)}}{100}

⇒ 1620 = 16a + 1800 – 18a

⇒ 1620 = 1800 – 2a

⇒ 2a = 1800 - 1620

⇒ a = 1800−16202\dfrac{1800 - 1620}{2}

⇒ a = 1802\dfrac{180}{2}

⇒ a = 90

∴ Percentage of 816X = 90%

and

Percentage 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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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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