Quick Summary
This unit focuses mainly on groups 13 and 14. Learn periodic trends, anomalous behaviour, allotropy, hydrides, oxides, and industrially important compounds such as borax, diborane, silicones, and silicates.
Group 13 Highlights
- Boron is non-metallic; aluminium is metallic.
- Boron forms covalent compounds and electron-deficient molecules.
- Aluminium oxide and hydroxide are amphoteric.
Important Compounds
- Borax: used in borax-bead test and glass manufacture
- Diborane: electron-deficient molecule with bridge bonds
- Boric acid: weak monobasic Lewis acid
Group 14 Highlights
- Carbon shows catenation strongly.
- Carbon allotropes: diamond, graphite, fullerene
- Silicon forms silicates, silicones, and silica-based materials.
Important Comparisons
| Property | Carbon | Silicon |
|---|---|---|
| Catenation | very high | lower |
| Multiple bonding | common | less effective |
| Common dioxide | CO2 molecular gas | SiO2 giant covalent solid |
Key Reactions to Know
Borax on heating (bead test):
Na₂B₄O₇·10H₂O → Na₂B₄O₇ → 2NaBO₂ + B₂O₃ (glassy bead)
Boric acid on heating:
H₃BO₃ →(373 K) HBO₂ →(red heat) B₂O₃
Diborane preparation:
2BF₃ + 6NaH → B₂H₆ + 6NaF
Diborane combustion (very exothermic):
B₂H₆ + 3O₂ → B₂O₃ + 3H₂O
Amphoteric alumina:
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O (acts as base)
Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O (acts as acid)
Structure Notes
- Diborane (B₂H₆): has two 3-centre-2-electron (banana) bridge bonds; 4 terminal H are normal bonds, 2 bridging H link the borons.
- Silicones: repeating R₂SiO units — water-repellent, heat-stable; used in sealants, lubricants, water-proofing.
Important Questions
One Mark
1. Why is diborane electron deficient? 2. Name one allotrope of carbon. 3. Write one use of borax. 4. What is catenation? 5. Name the linkage present in silicones.
Short Answer
1. Explain anomalous behaviour of boron. 2. Compare carbon dioxide and silicon dioxide. 3. Write a note on amphoteric nature of aluminium oxide. 4. Explain structure or bonding feature of diborane.
Long Answer
1. Discuss important compounds of boron. 2. Explain allotropy of carbon with uses. 3. Write notes on silicates and silicones.
Common Mistakes
- Treating boron compounds as ordinary ionic salts
- Forgetting bridge bonds in diborane
- Saying
SiO2is molecular likeCO2 - Confusing silicone polymer with silicon element
Revision Checklist
- [ ] Group 13 and 14 trends
- [ ] Boron anomaly
- [ ] Borax, boric acid, diborane
- [ ] Carbon allotropes
- [ ] Silicates and silicones
Frequently Asked Questions
Boron differs from the rest of group 13 because of its small size, high ionisation enthalpy, high electronegativity, and absence of d orbitals.