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Structure of the Atom — Question 6

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Question 6

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

Answer

The following rules are followed for writing the number of electrons in different energy levels or shells for the first eighteen elements:

  1. 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 = 2
    L shell, n=2, no. of electrons = 2 x 22 = 8
    M shell, n=3, no. of electrons = 2 x 32 = 18
  2. Electrons are not accommodated in a given shell, unless the inner shells are filled. Shells are filled in step-wise manner.
  3. 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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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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