Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Chapter Introduction
The p-Block Elements (Groups 13 and 14) form the bridge between the s-block and the rest of the p-block in the periodic table. Group 13 elements (boron family) and Group 14 elements (carbon family) show fascinating trends, anomalies, and diverse chemical behaviour. Boron, the first member of Group 13, is a metalloid with predominantly covalent chemistry, while its heavier congeners are metals. Carbon, the first member of Group 14, displays unparalleled catenation and forms the backbone of organic chemistry. This chapter is part of the Inorganic Chemistry unit and carries significant weight in CBSE exams — expect questions on boron's anomalous behaviour, aluminium extraction, carbon allotropes, and silicate structures.
Key Definitions
| Term | Definition |
|---|---|
| p-Block elements | Elements in which the last electron enters the p-orbitals of the valence shell (Groups 13–18) |
| Catenation | The ability of an element to form chains of bonds with atoms of the same element |
| Allotropy | Existence of an element in two or more different physical forms in the same state |
| Inert pair effect | Tendency of ns² electrons to remain unshared in compounds of heavier p-block elements |
| Electron-deficient compound | A compound in which the central atom has fewer electrons than needed for a noble gas configuration (e.g., B₂H₆) |
| Three-centre two-electron (3c-2e) bond | A bond involving three atoms but only two electrons, found in diborane |
| Borax | Sodium tetraborate decahydrate, Na₂B₄O₇·10H₂O, used in the borax bead test |
| Silicones | Organosilicon polymers with a —Si—O—Si— backbone and organic groups attached to silicon |
| Zeolites | Aluminosilicates with a three-dimensional network of SiO₄ and AlO₄ tetrahedra forming cavities/pores |
| Cryolite | Na₃AlF₆, used as a flux in aluminium extraction to lower the melting point of alumina |
Group 13: The Boron Family
Electronic Configuration
All Group 13 elements have the general valence shell configuration ns²np¹.
| Element | Atomic No. | Electronic Configuration | Atomic Radius (pm) |
|---|---|---|---|
| Boron (B) | 5 | [He] 2s² 2p¹ | 85 |
| Aluminium (Al) | 13 | [Ne] 3s² 3p¹ | 143 |
| Gallium (Ga) | 31 | [Ar] 3d¹⁰ 4s² 4p¹ | 135 |
| Indium (In) | 49 | [Kr] 4d¹⁰ 5s² 5p¹ | 167 |
| Thallium (Tl) | 81 | [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹ | 170 |
Note: The atomic radius of Ga (135 pm) is less than that of Al (143 pm) due to the poor shielding effect of d-electrons (d-block contraction).
Property Trends in Group 13
| Property | Trend | Reason |
|---|---|---|
| Atomic radius | Increases down the group (exception: Ga < Al) | Addition of new shells; d-block contraction in Ga |
| Ionisation enthalpy | Generally decreases down the group | Increase in atomic size; note: IE₁ of Ga > IE₁ of Al due to d-block contraction |
| Electronegativity | Decreases from B to Al, then increases slightly | Complex interplay of atomic size and nuclear charge |
| Oxidation states | +3 for B, Al; +1 becomes more stable down the group for Tl | Inert pair effect in heavier elements |
| Nature of oxides | B₂O₃ (acidic), Al₂O₃ (amphoteric), others (basic) | Change from non-metallic to metallic character |
Anomalous Properties of Boron
Boron differs significantly from the rest of Group 13 because of: 1. Smallest atomic size in the group (85 pm) 2. Highest ionisation enthalpy (much higher than Al) 3. Highest electronegativity (2.0 on Pauling scale) 4. Absence of d-orbitals — cannot expand octet 5. Only 6 valence electrons — forms electron-deficient compounds 6. Predominantly covalent bonding (unlike ionic compounds of other members)
These properties make boron's chemistry resemble silicon (diagonal relationship) more than aluminium.
Important Boron Compounds
Borax (Na₂B₄O₇·10H₂O)
- White crystalline solid, soluble in water
- On heating: Na₂B₄O₇·10H₂O → Na₂B₄O₇ → NaBO₂ + B₂O₃ (transparent bead)
- Borax bead test: Forms coloured metaborates with transition metal ions
| Metal Oxide | Bead Colour (Oxidising Flame) | Bead Colour (Reducing Flame) |
|---|---|---|
| CuO | Blue | Red (Cu) |
| Cr₂O₃ | Green | Green |
| MnO₂ | Violet | Colourless |
| CoO | Blue | Blue |
| Fe₂O₃ | Yellow-brown | Green |
| NiO | Brown | Brown |
- Borax solution is alkaline in water (acts as a buffer around pH 9)
- Uses: antiseptic, soldering flux, glaze in ceramics
Orthoboric Acid (H₃BO₃)
- White crystalline solid, soapy to touch
- Preparation: From borax — Na₂B₄O₇ + 2HCl + 5H₂O → 2NaCl + 4H₃BO₃
- Monobasic acid — acts as a Lewis acid, NOT a proton donor:
H₃BO₃ + H₂O ⇌ [B(OH)₄]⁻ + H⁺
- On heating above 370 K: H₃BO₃ → HBO₂ (metaboric acid) → B₂O₃ (boron trioxide)
- Uses: mild antiseptic, eye wash, food preservative
Diborane (B₂H₆)
- Simplest boron hydride, colourless gas, highly toxic
- Structure: Two boron atoms are linked by two bridging hydrogen atoms (banana bonds / 3c-2e bonds)
- Terminal B—H bonds are normal covalent bonds (2c-2e)
- Bridging B—H—B bonds are three-centre two-electron bonds (3c-2e)
- Each boron is approximately sp³ hybridised
- Four terminal H atoms and two boron atoms lie in the same plane; two bridging H atoms lie above and below this plane
- Electron count: 12 valence electrons (3 from each B + 1 from each H = 12), but only 4 conventional bonds — thus electron-deficient
Aluminium
Extraction by Hall-Héroult Process
1. Purification of bauxite (Al₂O₃·2H₂O): Bayer process — bauxite is treated with NaOH to form soluble sodium aluminate, then precipitated as Al(OH)₃ and heated to get pure Al₂O₃ 2. Electrolytic reduction:
- Electrolyte: Alumina dissolved in molten cryolite (Na₃AlF₆) at ~1173 K
- Cryolite lowers the melting point from 2323 K to ~1173 K and increases conductivity
- Cathode (steel vessel lined with carbon): Al³⁺ + 3e⁻ → Al(l)
- Anode (graphite rods): C + O²⁻ → CO + 2e⁻; 2C + 2O²⁻ → 2CO + 4e⁻
- Molten aluminium collects at the bottom and is tapped off
Properties and Uses of Aluminium
| Property | Description |
|---|---|
| Appearance | Silvery white metal, lightweight |
| Density | 2.7 g cm⁻³ (light metal) |
| Malleability | High — used in foil |
| Conductivity | Good conductor of heat and electricity |
| Reactivity | Reacts with dilute HCl and NaOH (amphoteric) |
| Passivation | Forms a protective Al₂O₃ layer on exposure to air |
| Reaction | Equation |
|---|---|
| With HCl | 2Al + 6HCl → 2AlCl₃ + 3H₂ |
| With NaOH | 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂ |
| With steam | 2Al + 3H₂O(g) → Al₂O₃ + 3H₂ |
| With O₂ | 4Al + 3O₂ → 2Al₂O₃ |
| Thermite reaction | 2Al + Fe₂O₃ → 2Fe + Al₂O₃ |
Group 14: The Carbon Family
Electronic Configuration
All Group 14 elements have the general valence shell configuration ns²np².
| Element | Atomic No. | Electronic Configuration | Atomic Radius (pm) |
|---|---|---|---|
| Carbon (C) | 6 | [He] 2s² 2p¹ | 77 |
| Silicon (Si) | 14 | [Ne] 3s² 3p² | 118 |
| Germanium (Ge) | 32 | [Ar] 3d¹⁰ 4s² 4p² | 123 |
| Tin (Sn) | 50 | [Kr] 4d¹⁰ 5s² 5p² | 140 |
| Lead (Pb) | 82 | [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p² | 146 |
Property Trends in Group 14
| Property | Trend | Reason |
|---|---|---|
| Atomic radius | Increases down the group | Addition of new shells |
| Ionisation enthalpy | Decreases down the group | Atomic size increases |
| Electronegativity | Decreases from C to Pb | Increased shielding |
| Metallic character | Increases down the group (C: non-metal → Pb: metal) | Decreasing IE, increasing size |
| Oxidation states | +4 and +2; +2 becomes more stable down the group | Inert pair effect |
| Catenation | Decreases from C to Pb | M—M bond strength decreases (C—C: 348 kJ/mol >> Pb—Pb: ) |
Covalent Bonding and Catenation
Catenation is the self-linking property of atoms. Carbon shows maximum catenation because:
- C—C bond energy (348 kJ mol⁻¹) is very high
- Small atomic size allows effective overlap of orbitals
- Carbon forms strong bonds with H, O, N, S, and halogens
Catenation tendency: C >> Si > Ge ≈ Sn >> Pb
Silicon shows limited catenation (Si—Si bond energy: 297 kJ mol⁻¹). Compounds with Si—Si bonds up to Si₈ exist but are unstable.
Allotropes of Carbon
| Property | Diamond | Graphite | Fullerene (C₆₀) |
|---|---|---|---|
| Hybridisation | sp³ | sp² | sp² |
| Structure | 3D network of tetrahedral C atoms | Layered hexagonal sheets | Spherical cage (truncated icosahedron) |
| Bonding | Each C bonded to 4 other C atoms | Each C bonded to 3 C atoms; one electron delocalised | Each C bonded to 3 C atoms; alternating single and double bonds |
| Hardness | Hardest natural substance | Soft, slippery | Intermediate |
| Electrical conductivity | Insulator | Good conductor | Semiconductor |
| Density | 3.51 g cm⁻³ | 2.22 g cm⁻³ | 1.65 g cm⁻³ |
| Appearance | Colourless, transparent | Grey-black, opaque | Yellow solid |
| Uses | Cutting, drilling, jewellery | Lubricant, electrodes, pencils | Superconductors, drug delivery, catalysts |
Graphite as a conductor: Each carbon is sp² hybridised. The unhybridised p-orbital on each C overlaps with adjacent p-orbitals forming a delocalised π-electron cloud above and below each layer. These delocalised electrons make graphite a good conductor in the plane of the layers.
Fullerene C₆₀: Contains 20 hexagons and 12 pentagons. Discovered by Kroto, Smalley, and Curl (Nobel Prize, 1996). Also called buckminsterfullerene or buckyball.
Silicones
- Organosilicon polymers with repeating —Si—O—Si— backbone
- General formula: [R₂SiO]ₙ (R = CH₃, C₆H₅, etc.)
- Preparation: Hydrolysis of chlorosilanes followed by condensation polymerisation
CH₃Cl + Si → CH₃SiCl₃ (methyl chlorosilane)
Hydrolysis: CH₃SiCl₃ + 3H₂O → CH₃Si(OH)₃ + 3HCl
Condensation: n CH₃Si(OH)₃ → cross-linked silicone
| Property of Silicones | Use |
|---|---|
| Water-repellent | Waterproof fabrics, sealants |
| Heat-resistant (up to 300°C) | High-temperature lubricants |
| Electrical insulator | Insulation in electrical appliances |
| Chemically inert | Biomedical implants, contact lenses |
| Low surface tension | Anti-foaming agent |
Silicates
Silicates are compounds derived from silicic acid (Si(OH)₄). They contain the fundamental building block SiO₄⁴⁻ (orthosilicate tetrahedron).
| Silicate Type | SiO₄ Sharing | Structure | Example |
|---|---|---|---|
| Orthosilicate (Nesosilicate) | No sharing | Discrete SiO₄⁴⁻ tetrahedra | Zircon (ZrSiO₄), Olivine (Mg₂SiO₄) |
| Pyrosilicate (Sorosilicate) | One O shared (2 tetrahedra) | Si₂O₇⁶⁻ units | Thortveitite (Sc₂Si₂O₇) |
| Ring silicates | Two O shared per tetrahedron | Cyclic — (Si₃O₉)⁶⁻ or (Si₆O₁₈)¹²⁻ | Beryl (Be₃Al₂Si₆O₁₈) |
| Chain silicates | Two O shared (single chain) or 2.5 O shared (double chain) | Linear chains — (SiO₃)²⁻ₙ or double chain — (Si₄O₁₁)⁶⁻ₙ | Pyroxenes (single), Amphiboles (double) |
| Sheet silicates | Three O shared per tetrahedron | 2D sheets — (Si₂O₅)²⁻ₙ | Talc, Mica, Kaolin |
| 3D Framework silicates | All four O shared | 3D network — SiO₂ (quartz) | Quartz, Feldspar, Zeolites |
Key fact: When Al³⁺ replaces Si⁴⁺ in a framework silicate, the structure acquires a negative charge that must be balanced by cations (Na⁺, K⁺, Ca²⁺). This gives rise to feldspars and zeolites.
Zeolites
- Aluminosilicates with a 3D network containing pores and cavities of molecular dimensions
- General formula: Mₓ/n[(AlO₂)ₓ(SiO₂)ᵧ]·mH₂O (M = metal cation)
- Used as catalysts in petrochemical cracking (e.g., ZSM-5)
- Used as ion exchangers in water softening
- Used as molecular sieves — selectively adsorb molecules based on size
- Shape-selective catalysis: zeolites with specific pore sizes allow only molecules of certain dimensions to enter/exit
Important Reactions
Boron Reactions
| Reaction | Equation |
|---|---|
| Borax bead formation | Na₂B₄O₇ → 2NaBO₂ + B₂O₃ (on heating) |
| Borax bead test (with CuO) | CuO + B₂O₃ → Cu(BO₂)₂ (blue bead) |
| Boric acid with water (Lewis acid) | H₃BO₃ + H₂O ⇌ [B(OH)₄]⁻ + H⁺ |
| Boric acid heating | H₃BO₃ —(370 K)→ HBO₂ —(red heat)→ B₂O₃ |
| Diborane combustion | B₂H₆ + 3O₂ → B₂O₃ + 3H₂O |
| Diborane hydrolysis | B₂H₆ + 6H₂O → 2B(OH)₃ + 6H₂ |
Aluminium Reactions
| Reaction | Equation |
|---|---|
| With HCl | 2Al + 6HCl → 2AlCl₃ + 3H₂ |
| With NaOH (amphoteric) | 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂ |
| Thermite reaction | 2Al + Fe₂O₃ → Al₂O₃ + 2Fe + heat |
| Passivation | 4Al + 3O₂ → 2Al₂O₃ (protective layer) |
| Hall-Héroult cathode | Al³⁺(melt) + 3e⁻ → Al(l) |
| Hall-Héroult anode | C(s) + 2O²⁻ → CO₂(g) + 4e⁻ |
Carbon Compound Reactions
| Reaction | Equation |
|---|---|
| CO formation (incomplete combustion) | 2C + O₂ → 2CO |
| CO₂ formation (complete combustion) | C + O₂ → CO₂ |
| CO as reducing agent | Fe₂O₃ + 3CO → 2Fe + 3CO₂ |
| Water-gas reaction | C + H₂O → CO + H₂ |
| Producer gas | 2C + O₂ + 4N₂ → 2CO + 4N₂ |
Common Mistakes to Avoid
1. Confusing borax bead test colours: Always specify whether the observation is in the oxidising or reducing flame — the same metal can give different colours (e.g., Cu gives blue in oxidising, red in reducing). 2. Forgetting boron's Lewis acid behaviour: H₃BO₃ is NOT a proton-donor acid. It acts as a Lewis acid by accepting OH⁻ from water. Do not write H₃BO₃ → H⁺ + H₂BO₃⁻. 3. Incorrect diborane structure: Do not draw conventional 2c-2e bonds for all B—H bonds. The two bridging H atoms form 3c-2e (banana) bonds — this is the exam-favourite question. 4. Confusing silicate types: Remember the sharing pattern — orthosilicates share 0 oxygen atoms, pyrosilicates share 1, chain silicates share 2, sheet silicates share 3, framework silicates share all 4 oxygens. 5. Forgetting the Ga anomaly: Gallium has a smaller atomic radius than aluminium due to d-block contraction. Do not assume a simple increase in radius down the group. 6. Writing CO as a Lewis base: CO is both a Lewis base (donates lone pair on C) and a Lewis acid (accepts into empty π* orbital). For CBSE, emphasise the basic character (donation to metals). 7. Confusing zeolites with regular silicates: Zeolites are specifically framework aluminosilicates with pores of molecular dimensions — not all framework silicates are zeolites.
Exam Tips
- Most-asked questions: Anomalous behaviour of boron (3-mark), borax bead test (2-mark), Hall-Héroult process (3-mark), allotropes of carbon (3-mark), silicate types with examples (2-mark)
- Draw structures: Practise the diborane structure with 3c-2e bonds — this is almost guaranteed to appear
- Diagonal relationship: Boron and silicon share similar properties (metallic character, covalent bonding, electrical insulators) — mention this for bonus marks
- Cryolite role: Always mention TWO functions — lowers melting point AND increases conductivity
- Silicate flowchart: Memorise the "number of shared oxygen atoms" approach: 0→neso, 1→soro, 2→ino (chain), 3→phyllo (sheet), 4→tecto (framework)
- Carbon vs silicon catenation: Always give bond energies (C—C: 348 kJ/mol vs Si—Si: 297 kJ/mol) when explaining why carbon shows greater catenation
- Inert pair effect: In Group 14, +2 oxidation state becomes more stable than +4 down the group — Sn²⁺ and Pb²⁺ are more stable than Sn⁴⁺ and Pb⁴⁺
Quick Revision Notes
Group 13 (Boron Family)
- Valence config: ns²np¹; +3 and +1 oxidation states
- Boron: anomalous (small size, high IE, covalent, electron-deficient)
- Borax: Na₂B₄O₇·10H₂O; bead test for metal ions
- Boric acid: H₃BO₃; Lewis acid (accepts OH⁻); monobasic
- Diborane: B₂H₆; two 3c-2e banana bonds; four terminal B—H bonds
- Aluminium: extracted by Hall-Héroult process using cryolite; amphoteric; passivated by Al₂O₃
Group 14 (Carbon Family)
- Valence config: ns²np²; +4 and +2 oxidation states
- Catenation: C >> Si > Ge > Sn (C—C bond strongest)
- Carbon allotropes: Diamond (sp³, hard, insulator), Graphite (sp², conductor, layered), Fullerene C₆₀ (cage)
- Silicones: —Si—O—Si— backbone; water-repellent, heat-resistant
- Silicates: built from SiO₄⁴⁻ tetrahedra; classified by shared O atoms (0→4)
- Zeolites: porous aluminosilicates; used as catalysts and molecular sieves
Key Reaction Chains
- Borax → NaBO₂ + B₂O₃ (bead) → Coloured metaborate with metal oxides
- Al₂O₃ → dissolved in cryolite → electrolysed → Al (Hall-Héroult)
- H₃BO₃ + H₂O → [B(OH)₄]⁻ + H⁺ (Lewis acid behaviour)