Question 1
How will you find the valency of chlorine, sulphur and magnesium?
(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 = 2
Hence, the valency of Mg is 2.
Chapter 4: Structure of the Atom — Quick Reference
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⁻
- 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.
- 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