← Chemistry — Std 11
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Elements of Groups 1, 2 and 13 to 18

Ch. 8Std 11

Easy Overview

This chapter dives into the s-block (Groups 1-2) and p-block (Groups 13-18) elements — the main group elements. From the explosive reactivity of alkali metals to the inertness of noble gases, from carbon's versatility to lead's toxicity, this covers the representative elements. Group 1 (alkali metals): ns¹, highly reactive, stored under kerosene. Reactivity increases down the group. Group 2 (alkaline earth metals): ns², harder than Group 1. Groups 13-18 show a transition from metallic to non-metallic character moving right. Group 14 features carbon — the element of life. Group 17 halogens are highly reactive non-metals. Group 18 noble gases were thought inert until Xe compounds were discovered in 1962.

Alkali Metals (Group 1) — General Properties

ns¹, silvery-white, soft (cut with knife), low MPs (Li 180, Na 98, K 63°C). Most electropositive metals, low IE ↓ down group. Form +1 ions. Reactivity with water ↑ down: Li (slow), Na (vigorous), K (catches fire), Rb/Cs (explode). With O₂: Li → Li₂O (oxide), Na → Na₂O₂ (peroxide), K → KO₂ (superoxide). Stored under kerosene. Flame: Li (crimson), Na (yellow), K (lilac).

Anomalous Properties of Lithium

Li differs due to small size, high IE, high EN, no d orbitals. Harder, higher MP. Reacts slowly with water. Forms Liâ‚‚O (not peroxide/superoxide). LiOH decomposes on heating (others stable). LiCl covalent (soluble in organic solvents). Diagonal relationship with Mg: both form nitrides, less soluble carbonates/phosphates, covalent organometallics.

Sodium and Its Important Compounds

NaOH (caustic soda): chlor-alkali process (electrolysis of NaCl). Strong base, used in soap, paper. Na₂CO₃ (washing soda): Solvay process. Used in glass, soap, water softening. NaHCO₃ (baking soda): decomposes on heating → Na₂CO₃ + CO₂ + H₂O. Used in baking (CO₂ makes dough rise) and antacids. NaCl: table salt, preservative, raw material.

Alkaline Earth Metals (Group 2)

ns², harder, higher MPs than Group 1. Form +2 ions. Reactivity ↑ down (Be inert, Mg slow with hot water, Ca vigorous). Oxides MO: basic (BeO amphoteric). Hydroxides M(OH)₂: less basic than Group 1, solubility ↑ down. Carbonates/sulfates: solubility trends opposite. Flame: Ca (brick red), Sr (crimson), Ba (apple green). Be, Mg none (electrons too tightly bound).

Calcium and Its Important Compounds

CaO (quicklime): CaCO₃→CaO+CO₂. Reacts with H₂O → Ca(OH)₂ (slaked lime). Used in cement, glass, steel. Ca(OH)₂: whitewash, mortar, bleaching powder. CaCO₃: limestone, marble, chalk. Decomposes on heating. CaSO₄·2H₂O (gypsum): heat 120°C → CaSO₄·½H₂O (plaster of Paris). CaOCl₂ (bleaching powder): Cl₂ + Ca(OH)₂. Mg: alloys (duralumin), flashbulbs, reducing agent for Ti/U.

Group 13 (Boron Family)

ns²np¹. B: metalloid, covalent network, high MP (2076°C). Al: most abundant metal in crust (8.3%), protective Al₂O₃ layer, corrosion-resistant. Ga: melts in hand (30°C), widest liquid range. In: ITO (touch screens). Tl: toxic. Inert pair effect: Tl⁺ > Tl³⁺. B anomalous: electron-deficient (BH₃ dimerizes to B₂H₆), Lewis acid.

Aluminum and Its Compounds

Hall-Héroult process: electrolysis of Al₂O₃ in molten cryolite (Na₃AlF₆). Amphoteric (reacts with both acids and bases). AlCl₃: Lewis acid, dimer Al₂Cl₆ in vapor phase, Friedel-Crafts catalyst. Alums: M⁺M³⁺(SO₄)₂·12H₂O — potash alum KAl(SO₄)₂·12H₂O used in water purification (coagulation), dyeing, styptic. Anodizing: electrolytic Al₂O₃ coating. Uses: aircraft, cans, power lines.

Group 14 (Carbon Family)

ns²np². C: non-metal, Si/Ge: metalloids, Sn/Pb: metals. +4 to +2 trend (inert pair effect). C favors +4; Pb favors +2 (Pb⁴⁺ strongly oxidizing). C unique: catenation (long chains), strong π bonds (C=C, C≡C), hybrids sp³/sp²/sp. Si: strong Si-O bonds (silicates, silicones). Allotropy: C (diamond, graphite, fullerenes, graphene), Si, Sn (white/gray).

Carbon — The Element of Life

Catenation: C-C bond (348 kJ/mol) stable but not too stable — perfect for chains. Multiple bonding: C=C (614), C≡C (839). Bonds with H, O, N, S, halogens, metals. Isomerism. Allotropes: diamond (sp³, hardest, insulator), graphite (sp², soft, conductor, lubricant), fullerenes (C₆₀), carbon nanotubes, graphene (single-layer graphite). Identical composition, vastly different properties.

Group 15 (Nitrogen Family)

ns²np³. N₂: gas (N≡N, 941 kJ/mol — very stable). NH₃: Haber process (N₂+3H₂⇌2NH₃, Fe catalyst, 450°C, 200 atm). P: white P₄ (ignites in air, stored under water), red P (stable), black P (most stable). +3 stable down group (inert pair): Bi³⁺ > Bi⁵⁺. H₃PO₄: phosphoric acid, used in fertilizers (superphosphate).

Group 16 (Oxygen Family)

ns²np⁴. O₂: essential for respiration. O₃ (ozone): protects from UV (stratosphere), pollutant (ground level). S: yellow S₈ rings, allotropes rhombic (stable <95.6°C) and monoclinic. H₂S: toxic, rotten egg. SO₂: from burning fossil fuels → acid rain. H₂SO₄ (king of chemicals): Contact process (S→SO₂→SO₃→H₂SO₄, V₂O₅ catalyst). Most produced industrial chemical.

Group 17 (Halogens)

ns²np⁵, diatomic (X₂). States: F₂, Cl₂ (gas), Br₂ (liquid), I₂ (solid). Color intensity: F₂ (pale yellow), Cl₂ (greenish-yellow), Br₂ (red-brown), I₂ (violet). Reactivity ↓ down: F₂ most reactive (attacks glass, water, everything). Displacement: F₂ displaces all; Cl₂ displaces Br⁻, I⁻; Br₂ displaces I⁻. HX acidity ↑ down: HF (weak), HI (very strong). Bond strength ↓ down.

Hydrogen Chloride and Chlorine Compounds

HCl: colorless gas, pungent, highly water-soluble (hydrochloric acid). Manufactured: H₂+Cl₂→2HCl or byproduct. Uses: cleaning metals, pH control. Cl₂: from chlor-alkali. Uses: water disinfection, bleaching, PVC, pesticides. Bleaching powder (CaOCl₂): Cl₂+Ca(OH)₂. Hypochlorite (OCl⁻) is the active bleaching agent — oxidizes colored substances. CHCl₃ (chloroform): anesthetic (replaced). CCl₄: solvent (restricted — toxicity, ozone depletion).

Group 18 (Noble Gases)

ns²np⁶, colorless, odorless, monatomic, very low reactivity. Discovered from air (Rayleigh, Ramsay). He: natural gas, radioactive decay. Others: fractional distillation of liquid air. Uses: He (balloons, MRI cooling), Ne (neon signs, red-orange), Ar (welding, inert atmosphere), Kr (high-intensity lamps), Xe (flash lamps, anesthesia). Xe compounds: Bartlett (1962) — XePtF₆. XeF₂, XeF₄, XeF₆ from Xe+F₂. XeO₃ explosive. Ended 'inert gas' notion.

Diagonal Relationship and Anomalous Properties

First element of each group often anomalous (small size, high EN, no d orbitals). Li-Mg: both form nitrides, carbonates decompose on heating, chlorides covalent. Be-Al: both amphoteric, covalent compounds, acid-resistant. B-Si: both metalloids, electron-deficient hydrides, acidic oxides. Similarities from comparable charge/size ratios.

Allotropy in Elements

Carbon: diamond (sp³, hardest, insulator), graphite (sp², soft, conductor, lubricant), fullerenes (C₆₀), graphene. Silicon: crystalline (diamond-like, semiconductor) and amorphous. Phosphorus: white P₄ (ignites in air, stored under water), red P (polymeric, stable), black P (most stable, layered). Sulfur: rhombic (α, S₈, stable <95.6°C) and monoclinic (β, stable >95.6°C). Tin: white β (metallic, room T), gray α (powdery, <13.2°C) — tin pest.

Industrially Important Compounds

H₂SO₄: Contact process. Uses: fertilizers, refining, batteries. HNO₃: Ostwald process (NH₃→NO→NO₂→HNO₃). Uses: fertilizers, explosives (TNT), dyes. Na₂CO₃: Solvay process. NH₃: Haber process (N₂+3H₂⇌2NH₃). Uses: fertilizers, HNO₃, explosives, refrigerants. CaOCl₂: bleaching, disinfection. Na₂CO₃·10H₂O: washing soda.

Biological Importance of Main Group Elements

Na, K: nerve impulses, fluid balance. Ca: bones, muscle contraction, blood clotting. Mg: chlorophyll, ATP stabilization. P: DNA/RNA backbone, ATP. O: respiration. N: proteins, nucleic acids. S: amino acids (Cys, Met), disulfide bonds. Cl: electrolyte, stomach acid. I: thyroid hormones (thyroxine). F: tooth enamel (fluorapatite). B: plant cell walls. Si: connective tissues, diatom shells.

Key Points

  • •Alkali metals (Gr 1): ns¹, soft, +1, stored under kerosene, reactivity ↑ down
  • •Flame test: Li (crimson), Na (yellow), K (lilac), Rb (red-violet), Cs (blue)
  • •Anomalous Li: diagonal relationship with Mg — similar charge/size
  • •Alkaline earth (Gr 2): ns², +2, harder than Gr 1; Be anomalous — amphoteric, covalent
  • •Ca compounds: quicklime (CaO), slaked lime Ca(OH)â‚‚, gypsum CaSO₄·2Hâ‚‚O, POPS
  • •Bleaching powder: CaOClâ‚‚ (Clâ‚‚+Ca(OH)â‚‚); bleaching and disinfection
  • •Gr 13: B (metalloid) → Al, Ga, In, Tl (metals); inert pair: Tl⁺ > Tl³⁺
  • •Al: Hall-Héroult process, amphoteric, AlCl₃ Lewis acid, potash alum coagulant
  • •Gr 14: C catenation and multiple bonding → millions of compounds
  • •Allotropes: diamond (sp³, hardest), graphite (sp², soft conductor), fullerenes
  • •Inert pair effect in Gr 14: Pb²⁺ more stable than Pb⁴⁺
  • •Gr 15: Nâ‚‚ (stable), NH₃ (Haber), Pâ‚„ (white), P (red), Bi³⁺ stable
  • •Gr 16: Oâ‚‚ essential, O₃ UV protection, S₈ rings, Hâ‚‚SOâ‚„ (Contact process)
  • •Gr 17: Fâ‚‚ most reactive; Clâ‚‚ (disinfectant); Iâ‚‚ (antiseptic); HF weak acid, HI strong
  • •Gr 18: He, Ne, Ar, Kr, Xe, Rn — Xe forms compounds with F and O
  • •Diagonal: Li-Mg, Be-Al, B-Si — similar charge/size ratios
  • •Biological roles: Na/K (nerve), Ca (bones), Mg (chlorophyll), P (DNA), I (thyroid)

Practice Questions

  • Why are alkali metals stored under kerosene? Write Na + Hâ‚‚O and Na + Oâ‚‚ reactions.
  • Explain anomalous properties of Li and diagonal relationship with Mg.
  • Describe preparation, properties, uses of: (i) bleaching powder (ii) plaster of Paris.
  • Explain halogen reactivity trend down the group. Why is Fâ‚‚ most reactive?
  • Why does carbon form so many compounds? Explain catenation and multiple bonding.
  • Describe allotropes of carbon with structures and properties.
  • Explain inert pair effect in Groups 13, 14, 15.
  • Describe extraction of Al by Hall-Héroult process. Why is cryolite added?
  • Write reactions for Contact process (Hâ‚‚SOâ‚„ manufacture).
  • Why were noble gases considered inert? How was this disproved?