CDS Chemistry · Teaching notes
Chemical Bonding — CDS Chemistry
Chemical Bonding has 9 past-year questions in CDS General Knowledge, from 2016 (II) to 2025 (II), roughly one every year. It reaches further into Class 11 than most of CDS chemistry: hybridisation, molecular shapes, sigma and pi bonds, oxidation numbers. Each idea comes down to one counting rule (electron groups for shape, bonds for sigma and pi, the zero-sum rule for oxidation numbers), so learn the rule and the questions follow.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Ionic and Covalent Bonds
2 PYQsHow ionic bonds (electron transfer) differ from covalent bonds (electron sharing), what makes a compound more ionic, and why plastics last so long.
Molecular Shapes and Bond Counting
3 PYQsHow hybridisation sets a molecule's shape, from linear HCN to trigonal bipyramidal PCl₅, and how to count sigma and pi bonds.
Valency and Oxidation Numbers
4 PYQsThe rules for working out an oxidation number, why hydrogen and nitrogen take several, and which elements show more than one valency.
Formula & revision sheet
2 formulas · 3 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
2 formulas · 3 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (1)
Ionic and covalent bonds5 rows
| Property | Ionic | Covalent |
|---|---|---|
| How the bond forms | Electron transfer | Electron sharing |
| Typical partners | Metal + non-metal | Non-metal + non-metal |
| Melting point | High | Usually low |
| Conducts when molten | Yes | No |
| Example | K₂S (highly ionic) | SiO₂, NCl₃, plasticsQ |
Watch out for (3)
- A big, singly charged cation gives the most ionic compound→ Ionic and covalent bonds
- Plastics last because of covalent bonds→ Ionic and covalent bonds
- SiO₂ is covalent despite its high melting point→ Ionic and covalent bonds
Formulas (1)
Reference tables (1)
Hybridisation and molecular shape6 rows
| Molecule | Hybridisation | Shape |
|---|---|---|
| HCN | sp | Linear |
| HCHO | sp² | Trigonal planar |
| CH₃F | sp³ | Tetrahedral |
| NH₃ | sp³ (one lone pair) | Trigonal pyramidal |
| PCl₅ | sp³d (dsp³) | Trigonal bipyramidalQ |
| SF₆ | sp³d² | Octahedral |
Watch out for (4)
- Ammonia is a pyramid, not a tetrahedron→ Hybridisation and molecular shape
- Five groups need a d orbital→ Hybridisation and molecular shape
- Count the C–H bonds too→ Counting sigma and pi bonds
- Saturated means zero pi bonds→ Counting sigma and pi bonds
Formulas (1)
Reference tables (1)
Elements with more than one valency6 rows
| Element | Valencies or oxidation states | Examples |
|---|---|---|
| Phosphorus | 3 and 5 | PCl₃, PCl₅Q |
| Sulphur | 2, 4, 6 | H₂S, SO₂, SO₃ |
| Iron | +2 and +3 | FeSO₄, FeCl₃ |
| Copper | +1 and +2 | Cu₂O, CuO |
| Sodium, lithium | +1 only | NaCl, LiCl |
| Calcium | +2 only | CaCl₂ |
Watch out for (5)
- Hydrogen is not always +1→ Working out oxidation numbers
- N₂O₅ is +5, the highest for nitrogen→ Working out oxidation numbers
- Peroxide oxygen is −1→ Working out oxidation numbers
- Only the transition metal varies→ Elements with more than one valency
- Phosphorus is 3 and 5, not 3 and 4→ Elements with more than one valency
PYQ weightage by concept
5 concepts · 9 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
5 concepts · 9 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Hybridisation and molecular shape | 2 | 22% |
| Counting sigma and pi bonds | 1 | 11% |
| Concept | PYQs | Share |
|---|---|---|
| Working out oxidation numbers | 2 | 22% |
| Elements with more than one valency | 2 | 22% |
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.