Tamil Nadu Class 12 Chemistry p-Block Elements I — Samacheer Kalvi

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

Key Reactions to Know

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 SiO2 is molecular like CO2
• Confusing silicone polymer with silicon element

Revision Checklist

• [ ] Group 13 and 14 trends
• [ ] Boron anomaly
• [ ] Borax, boric acid, diborane
• [ ] Carbon allotropes
• [ ] Silicates and silicones