Introduction to p-Block Elements
Groups 13 to 18 of the periodic table constitute the p-block. The last electron enters the p-orbital. This chapter focuses on Groups 13 and 14.
General Characteristics
- General configuration:
ns^2 np^(1-6). - Includes metals, non-metals, and metalloids.
- Wide range of oxidation states.
- Covalent character increases across period and up a group.
Group 13: Boron Family
B, Al, Ga, In, Tl.
General Properties
- Electronic configuration:
ns^2 np^1. - Common oxidation state: +3.
- Tl shows +1 oxidation state (inert pair effect).
- Boron is a metalloid; others are metals.
Trends
| Property | Down the Group |
|---|---|
| Atomic radius | Increases |
| Ionisation enthalpy | Decreases (but irregular) |
| Melting point | Decreases |
| Electronegativity | Decreases |
Anomalous Behaviour of Boron
- Non-metal (others are metals).
- Forms covalent compounds (small size, high charge).
- Does not form B^3+ ion.
- Shows allotropy (amorphous and crystalline).
- Forms boron hydrides (boranes).
Important Compounds of Boron
Borax (Na2B4O7.10H2O):
- On heating:
Na2B4O7 -> 2NaBO2 + B2O3. - B2O3 with CoO/NiO gives coloured beads (borax bead test).
- Uses: In glass, enamel, as flux.
Boric Acid (H3BO3):
- Weak monobasic acid (acts as Lewis acid).
B(OH)3 + H2O -> [B(OH)4]^- + H+.- Uses: Antiseptic, eyewash, flame retardant.
Aluminium
- Most abundant metal in Earth's crust.
- Amphoteric: reacts with acids and bases.
2Al + 6HCl -> 2AlCl3 + 3H2.2Al + 2NaOH + 2H2O -> 2NaAlO2 + 3H2.- Used in alloys, foils, utensils, electrical cables.
- Al2O3 (alumina): high melting point, used as abrasive.
Group 14: Carbon Family
C, Si, Ge, Sn, Pb.
General Properties
- Electronic configuration:
ns^2 np^2. - Common oxidation states: +4, +2.
- Inert pair effect: stability of +2 increases down the group.
Trends
| Property | Down the Group |
|---|---|
| Atomic radius | Increases |
| Ionisation enthalpy | Decreases |
| Metallic character | Increases |
| Maximum covalency | 4 (but Si can expand octet) |
Allotropes of Carbon
Diamond:
- sp^3 hybridised carbon.
- Tetrahedral structure, very hard.
- Electrical insulator, thermal conductor.
- High refractive index.
Graphite:
- sp^2 hybridised carbon.
- Hexagonal layered structure.
- Soft (layers slide), good conductor (delocalised electrons).
- Used as lubricant and in pencils.
Fullerenes:
- C60, C70, etc.
- Soccer ball shape (12 pentagons, 20 hexagons).
- Discovered in 1985 (Kroto, Smalley, Curl).
Carbon Compounds
CO2: Linear molecule. Dry ice (solid CO2) used as refrigerant. Greenhouse gas.
CO: Colourless, odourless, poisonous. Binds to haemoglobin (200x stronger than O2).
Silica and Silicates
SiO2 (Silica):
- Found as quartz, sand, rock crystal.
- Three-dimensional network solid.
- Used in glass manufacture.
Silicates:
- Basic structural unit:
SiO4^4-tetrahedron. - Types: orthosilicates, pyrosilicates, chain, sheet, cyclic silicates.
Silicon Compounds
Silicones:
- Organosilicon polymers.
R2SiOunits. - Water repellent, heat resistant.
- Used as sealants, lubricants, insulators.
Sodium silicate (Na2SiO3): Water glass, used in detergents and adhesives.
Worked Examples
Example 1: Explain why BCl3 is a Lewis acid while AlCl3 is also a Lewis acid but dimerises. Solution: B in BCl3 has 6 electrons (incomplete octet), accepts electron pair (Lewis acid). Al in AlCl3 also has incomplete octet but larger size allows dimerisation to Al2Cl6 (bridging Cl atoms).
Example 2: Why is CO2 a gas while SiO2 is a solid? Solution: CO2 has discrete molecular structure (weak van der Waals forces). SiO2 has 3D network covalent bonding (very strong).
Common Mistakes
- Inert pair effect: Down Group 13 and 14, lower oxidation state becomes more stable (+1 for Tl, +2 for Pb).
- Boron is electron deficient: Forms electron-deficient compounds like B2H6 (three-centre two-electron bonds).
- Catenation: Carbon shows maximum catenation due to strong C-C bonds.
- SiO2 vs CO2: CO2 is monomeric; SiO2 is polymeric network.
ISC Exam Focus
- Theory (70%): Group trends, boron compounds, allotropes of carbon, silicones.
- Application (30%): Explaining properties, writing reactions, comparing elements.
- ISC frequently asks: "Compare diamond and graphite" and "Explain inert pair effect".
- Borax bead test and boric acid properties are commonly tested.
Self-Test Questions
Q1: Write the electronic configuration of Group 13 and Group 14 elements. Answer: Group 13: ns^2 np^1. Group 14: ns^2 np^2.
Q2: Distinguish between diamond and graphite. Answer: Diamond: sp^3, hard, insulator, tetrahedral. Graphite: sp^2, soft, conductor, layered.
Q3: What is the inert pair effect? Give an example. Answer: Tendency of ns^2 electrons to remain unshared increases down a group. Example: Tl+ is more stable than Tl3+, Pb2+ more stable than Pb4+.
Q4: Write the structure of diborane (B2H6). Answer: Banana-shaped bonds with two 3-centre-2-electron B-H-B bridge bonds and four terminal B-H bonds.
Q5: Why is boric acid a weak monobasic acid?
Answer: It accepts OH- from water: B(OH)3 + H2O -> [B(OH)4]^- + H+, acting as Lewis acid.
Q6: Write two uses of silicones. Answer: As sealants, water repellents, lubricants, and electrical insulators.
