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Structure of the Atom — Question 10

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Question 10

If 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.

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 \times 49.7}{100} = 39.263 u

Isotope of bromine with atomic mass 81 u = 50.3%

Contribution of 3581Br to the atomic mass of bromine = 81×50.3100\dfrac{81 \times 50.3}{100} = 40.743u

Hence, the average atomic mass of the bromine atom = 39.263 + 40.743 = 80.006 u

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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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