Class 11 Notes

Class 11 Chemistry Notes Chapter 4 (The p- Block Elements) – Chemistry Part-II Book

Philoid

Philoid

— 10 min read

Chemistry Part-II
Alright class, let's begin our focused revision of Chapter 4, 'The p-Block Elements' from your Class 11 NCERT Part-II book. This is a crucial chapter for various government exams, focusing on Groups 13 and 14. Pay close attention to the trends, anomalies, and important compounds.

Chapter 4: The p-Block Elements (Class 11 Portion: Groups 13 & 14)

Introduction to p-Block Elements

  • Elements in which the last electron enters the outermost p orbital belong to the p-block.
  • They comprise Groups 13 to 18 of the periodic table.
  • Their valence shell electronic configuration is ns²np¹⁻⁶ (except for He, which is 1s²).
  • p-Block contains metals, non-metals, and metalloids, showing great diversity in properties.
  • The properties of p-block elements are significantly influenced by their atomic size, ionization enthalpy, electronegativity, and the presence/absence of d-orbitals in their valence shell.

Group 13 Elements: The Boron Family

  • Members: Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), Thallium (Tl).

  • Electronic Configuration: General configuration is ns²np¹.

  • Atomic and Ionic Radii:

    • Generally increase down the group from B to Tl.
    • Anomaly: Atomic radius of Ga (135 pm) is slightly less than that of Al (143 pm).
    • Reason: Presence of 10 d-electrons in Ga which offer poor shielding effect for the outer electrons from the increased nuclear charge.

  • Ionization Enthalpy (IE):

    • Does not decrease smoothly down the group.
    • ΔᵢH₁ (kJ mol⁻¹): B(801) > Tl(589) > Ga(579) > Al(577) > In(558).
    • The decrease from B to Al is expected. The discontinuity between Al and Ga is due to poor shielding by d-electrons in Ga. The discontinuity between In and Tl is due to poor shielding by 14 f-electrons in Tl (lanthanoid contraction effect).
    • Sum of first three IEs (ΔᵢH₁ + ΔᵢH₂ + ΔᵢH₃) is very high.

  • Electronegativity:

    • Decreases from B to Al, then increases slightly for Ga, and remains nearly constant for In and Tl. B(2.0) > Tl(1.8) ≈ In(1.7) ≈ Ga(1.6) > Al(1.5).
    • Irregular trend is again attributed to discrepancies in atomic size and shielding effects.

  • Physical Properties:

    • Boron is a non-metal (hard, black solid, high MP).
    • Others (Al, Ga, In, Tl) are soft metals with relatively low MPs (Ga has a very low MP of 303 K).
    • Density increases down the group.

  • Chemical Properties:

    • Oxidation States:
      • Common oxidation state is +3.
      • Tendency to show +1 oxidation state increases down the group (Al < Ga < In < Tl).
      • Inert Pair Effect: Reluctance of the ns² electrons to participate in bonding due to poor shielding by inner d and f electrons, making the heavier elements (like Tl) prefer the lower (+1) oxidation state. Tl⁺¹ is more stable than Tl⁺³.
    • Reactivity towards Air:
      • Boron is unreactive in crystalline form. Amorphous Boron reacts with air on heating to form B₂O₃.
      • Al forms a protective oxide layer (Al₂O₃).
      • All react with O₂ at high temperatures to form trioxides (M₂O₃).
      • Tl also forms Tl₂O.
      • Nature of Oxides: B₂O₃ (acidic), Al₂O₃ & Ga₂O₃ (amphoteric), In₂O₃ & Tl₂O₃ (basic).
    • Reactivity towards Acids and Alkalis:
      • Boron doesn't react with non-oxidizing acids (like HCl) but reacts with strong oxidizing acids (like conc. HNO₃, conc. H₂SO₄). Boron also reacts with fused alkali (NaOH/KOH).
      • Aluminium dissolves in mineral acids and aqueous alkalis (shows amphoteric character).
      • 2Al(s) + 6HCl(aq) → 2Al³⁺(aq) + 6Cl⁻(aq) + 3H₂(g)
      • 2Al(s) + 2NaOH(aq) + 6H₂O(l) → 2Na⁺[Al(OH)₄]⁻(aq) + 3H₂(g) (Sodium tetrahydroxoaluminate(III))
      • Ga shows similar amphoteric behaviour. In and Tl dissolve in acids but not usually in alkalis.
    • Reactivity towards Halogens:
      • Elements react with halogens (X₂) to form trihalides (MX₃).
      • Tl also forms stable TlX (Tl⁺¹ state).
      • Boron trihalides (BX₃) are covalent, electron-deficient molecules, acting as strong Lewis acids (accept electron pairs). Order of Lewis acidity: BI₃ > BBr₃ > BCl₃ > BF₃ (due to back-bonding in BF₃).
      • AlCl₃ exists as a dimer (Al₂Cl₆) in non-polar solvents and vapour phase, achieving stability. In solid state, it has an ionic lattice.

  • Anomalous Properties of Boron:

    • Small size, high IE, high electronegativity.
    • Non-metallic character.
    • Forms only covalent compounds.
    • Maximum covalency is 4 (due to absence of d-orbitals), e.g., in [BF₄]⁻. Other members can expand their covalency (e.g., [AlF₆]³⁻).
    • Boron oxide (B₂O₃) is acidic, while others are amphoteric or basic.
    • Boron halides (BX₃) are monomeric Lewis acids, while AlCl₃ dimerizes.

  • Important Compounds of Boron:

    • Borax (Sodium tetraborate decahydrate): Na₂B₄O₇·10H₂O
      • Actual formula: Na₂[B₄O₅(OH)₄]·8H₂O. Contains the tetranuclear unit [B₄O₅(OH)₄]²⁻.
      • Preparation: From colemanite ore (Ca₂B₆O₁₁) treated with Na₂CO₃.
      • Properties: White crystalline solid, soluble in water. Aqueous solution is alkaline due to hydrolysis.
      • Na₂B₄O₇ + 7H₂O → 2NaOH + 4H₃BO₃ (Orthoboric acid)
      • Action of Heat (Borax Bead Test): On heating, loses water, swells up, and finally forms a transparent glassy bead (sodium metaborate + boric anhydride).
      • Na₂B₄O₇·10H₂O --(Heat)--> Na₂B₄O₇ --(Strong Heat)--> 2NaBO₂ + B₂O₃ (Glassy bead)
      • Used to identify coloured metal ions (e.g., Co²⁺ gives blue bead - Co(BO₂)₂).
    • Orthoboric Acid: H₃BO₃ or B(OH)₃
      • Preparation: Acidifying aqueous solution of borax; Hydrolysis of boron compounds (halides, hydrides).
      • Properties: White crystalline solid, soapy touch, sparingly soluble in cold water, more soluble in hot water.
      • It is a weak monobasic Lewis acid, accepting OH⁻ from water: B(OH)₃ + H₂O ⇌ [B(OH)₄]⁻ + H⁺
      • Action of Heat: H₃BO₃ --(Heat, ~370K)--> HBO₂ (Metaboric acid) --(Heat, >410K)--> H₂B₄O₇ (Tetraboric acid) --(Red Heat)--> B₂O₃ (Boric anhydride)
      • Structure: Layered structure where planar BO₃ units are joined by hydrogen bonds.
    • Diborane: B₂H₆
      • Preparation: 2NaBH₄ + I₂ → B₂H₆ + 2NaI + H₂ ; 4BF₃ + 3LiAlH₄ → 2B₂H₆ + 3LiF + 3AlF₃ (Lab method)
      • Properties: Colourless, highly toxic gas, catches fire spontaneously in air (releases large energy). Readily hydrolysed by water.
      • B₂H₆ + 3O₂ → B₂O₃ + 3H₂O ; ΔH = -ve (large)
      • B₂H₆ + 6H₂O → 2H₃BO₃ + 6H₂
      • Reacts with Lewis bases (like NH₃, CO) undergoing cleavage reactions.
      • Structure: Contains two bridging hydrogen atoms (B-H-B bonds) which are 3-centre-2-electron bonds (banana bonds). The four terminal B-H bonds are normal 2-centre-2-electron bonds. Boron atoms are sp³ hybridized.

  • Uses:

    • Boron: Boron fibres (bullet-proof vests, aircraft parts), neutron absorber in nuclear reactors, semiconductor industry.
    • Borax/Boric Acid: Antiseptics, eye lotions, glass industry (pyrex glass), pottery glazes, washing powders.
    • Aluminium: Utensils, foils, construction (window frames), aircraft/automobile parts (alloys like duralumin, magnalium), electrical transmission wires, thermite welding.

Group 14 Elements: The Carbon Family

  • Members: Carbon (C), Silicon (Si), Germanium (Ge), Tin (Sn), Lead (Pb).

  • Electronic Configuration: General configuration is ns²np².

  • Covalent Radii: Increase down the group from C to Pb. Slight increase from Si to Pb compared to C to Si.

  • Ionization Enthalpy (IE):

    • Generally decreases down the group.
    • ΔᵢH₁: C > Si > Ge > Sn < Pb. The slight increase for Pb is due to poor shielding by inner d and f electrons.
    • Sum of first four IEs is very high.

  • Electronegativity: Values are C(2.5) > Si(1.8) ≈ Ge(1.8) ≈ Sn(1.8) < Pb(1.9). Carbon is significantly more electronegative. From Si to Sn, values are similar. Pb is slightly higher again due to ineffective shielding.

  • Physical Properties:

    • C (non-metal), Si & Ge (metalloids), Sn & Pb (metals).
    • Melting and Boiling points are generally high for C and Si, then decrease down the group.

  • Chemical Properties:

    • Oxidation States:
      • Common oxidation states are +4 and +2.
      • Tendency to show +2 oxidation state increases down the group due to the inert pair effect (C < Si < Ge < Sn < Pb). Pb²⁺ is more stable than Pb⁴⁺, while Sn²⁺ acts as a reducing agent (gets oxidized to Sn⁴⁺). Si only shows +4 reliably.
      • Compounds in +4 state are generally covalent (except SnF₄, PbF₄ which are ionic). Compounds in +2 state are generally ionic.
    • Reactivity towards Oxygen:
      • Form monoxides (MO) and dioxides (MO₂).
      • CO₂ (acidic), SiO₂ (acidic), GeO₂ (acidic).
      • SnO₂ (amphoteric), PbO₂ (amphoteric).
      • CO (neutral), GeO (acidic), SnO (amphoteric), PbO (amphoteric).
      • SiO only exists at high temperatures.
    • Reactivity towards Water:
      • C, Si, Ge are not affected by water.
      • Tin reacts with steam to produce SnO₂ and H₂.
      • Lead is unaffected by water, likely due to a protective oxide film.
    • Reactivity towards Halogens:
      • Form tetrahalides (MX₄) and dihalides (MX₂).
      • MX₄ are mostly covalent, sp³ hybridized (tetrahedral). Exceptions: SnF₄, PbF₄ (ionic).
      • Stability of tetrahalides decreases down the group. PbI₄ does not exist (Pb⁴⁺ oxidizes I⁻).
      • Stability of dihalides increases down the group (inert pair effect). GeX₂ < SnX₂ < PbX₂.
      • Hydrolysis: Most tetrahalides (except CCl₄) are readily hydrolysed by water because the central atom can accommodate lone pairs from water using vacant d-orbitals (Si, Ge, Sn, Pb). CCl₄ is not hydrolysed as carbon has no d-orbitals.
      • SiCl₄ + 4H₂O → Si(OH)₄ (Silicic acid) + 4HCl

  • Anomalous Behaviour of Carbon:

    • Catenation: Unique ability to form strong covalent bonds with itself (C-C) leading to long chains and rings. Order: C >> Si > Ge ≈ Sn. Pb shows negligible catenation. Reason: High C-C bond enthalpy.
    • pπ–pπ Multiple Bonding: Carbon readily forms stable multiple bonds (C=C, C≡C, C=O, C=N, C≡N) with itself and other small atoms due to its small size and high electronegativity, allowing effective sideways overlap of p-orbitals. Heavier elements do not form stable pπ–pπ bonds because their atomic orbitals are too large and diffuse for effective overlap.
    • Small size, high electronegativity, high ionization enthalpy.
    • Absence of d-orbitals limits its covalency to 4.

  • Allotropes of Carbon: Existence of an element in multiple forms having different physical properties but similar chemical properties.

    • Diamond:
      • Structure: Crystalline lattice, each C atom is sp³ hybridized and tetrahedrally bonded to four other C atoms. Rigid 3D network. C-C bond length 154 pm.
      • Properties: Hardest substance known, high MP, electrical insulator (no free electrons), thermal conductor, transparent, high refractive index.
    • Graphite:
      • Structure: Layered structure. Each layer consists of planar hexagonal rings. Each C atom is sp² hybridized and bonded to three other C atoms within the layer (C-C bond length 141.5 pm). Fourth valence electron is delocalized over the whole sheet (forms π system). Layers are held by weak van der Waals forces (distance 340 pm).
      • Properties: Soft, slippery (layers slide), good conductor of electricity (delocalized electrons), high MP, thermodynamically most stable allotrope. Used as lubricant, electrode material, pencil lead.
    • Fullerenes:
      • Structure: Cage-like molecules (e.g., C₆₀ - Buckminsterfullerene). Spherical shape with 20 six-membered rings and 12 five-membered rings. Each C atom is sp² hybridized. Looks like a soccer ball (Buckminsterfullerene).
      • Properties: Crystalline solids, covalent. C₆₀ is soluble in organic solvents.

  • Important Compounds of Carbon and Silicon:

    • Carbon Monoxide (CO):
      • Preparation: Incomplete combustion of carbon/fuels; Dehydration of formic acid with conc. H₂SO₄; Industrial: Passing steam over hot coke.
      • Properties: Colourless, odourless, highly poisonous gas (forms stable complex with haemoglobin). Powerful reducing agent (used in metallurgy). Burns with a blue flame. Acts as a ligand (metal carbonyls). Structure: C≡O (with a coordinate bond).
    • Carbon Dioxide (CO₂):
      • Preparation: Complete combustion of carbon/hydrocarbons; Action of dilute acids on metal carbonates; Industrial: Heating limestone.
      • Properties: Colourless, odourless gas. Acidic oxide (forms carbonic acid H₂CO₃ with water). Used in photosynthesis by plants. Solid CO₂ is Dry Ice (sublimes). Contributes to greenhouse effect. Structure: Linear molecule (O=C=O), C is sp hybridized.
    • Silicon Dioxide (Silica, SiO₂):
      • Occurrence: Quartz, cristobalite, tridymite (crystalline forms); Kieselguhr (amorphous).
      • Structure: Covalent, 3D network solid. Each Si atom is tetrahedrally bonded to 4 oxygen atoms, and each O atom is bonded to 2 Si atoms (Si-O-Si linkage). High Si-O bond enthalpy makes it very stable.
      • Properties: High MP, hard solid. Resistant to chemical attack (except by HF and NaOH). Acidic oxide.
      • SiO₂ + 2NaOH → Na₂SiO₃ + H₂O
      • SiO₂ + 4HF → SiF₄ + 2H₂O
      • Uses: Piezoelectric material (quartz), glass making, silica gel (dehydrating agent, chromatography), kieselguhr (filtration plants).
    • Silicones:
      • Organosilicon polymers containing repeating R₂SiO units held by Si-O-Si linkages. (-[R₂Si-O]-)n
      • Preparation: Hydrolysis of alkyl or aryl substituted chlorosilanes (e.g., R₂SiCl₂, R₃SiCl, RSiCl₃) followed by polymerization.
      • R₂SiCl₂ + 2H₂O → R₂Si(OH)₂ + 2HCl
      • n R₂Si(OH)₂ --(Polymerization)--> (-[R₂Si-O]-)n + nH₂O
      • Chain length/cross-linking controlled by adding R₃SiCl (chain stopper) or RSiCl₃ (cross-linking).
      • Properties: Water repellent, high thermal stability, good electrical insulators, chemically inert.
      • Uses: Sealants, greases, electrical insulators, water-proofing fabrics, surgical implants.
    • Silicates:
      • Minerals containing SiO₄⁴⁻ tetrahedral units. Si atom is sp³ hybridized.
      • Basic structural unit: [SiO₄]⁴⁻ tetrahedron.
      • Types (based on sharing of oxygen atoms): Orthosilicates (discrete [SiO₄]⁴⁻), Pyrosilicates ([Si₂O₇]⁶⁻ - one O shared), Cyclic silicates ([Si₃O₉]⁶⁻, [Si₆O₁₈]¹²⁻ - two O shared per tetrahedron), Chain silicates (Pyroxenes [SiO₃]n²ⁿ⁻ - two O shared; Amphiboles [Si₄O₁₁]n⁶ⁿ⁻ - alternate sharing of two and three O), Sheet silicates (Phyllosilicates [Si₂O₅]n²ⁿ⁻ - three O shared, e.g., talc, mica), 3D silicates (Tectosilicates [SiO₂]n - all four O shared, e.g., quartz, zeolites).
    • Zeolites:
      • Aluminosilicates: Some Si atoms in the 3D network of SiO₂ are replaced by Al³⁺ ions, creating a framework [AlO₄]⁵⁻ which requires balancing cations (like Na⁺, K⁺, Ca²⁺). General formula: Mₓ/ₙ[(AlO₂)ₓ(SiO₂)y]·zH₂O.
      • Structure: Honeycomb-like structure with pores/cavities of specific sizes.
      • Uses: Shape-selective catalysts in petrochemical industry (e.g., ZSM-5 converts alcohols to gasoline), ion exchangers (softening of hard water), adsorbents.

  • Uses:

    • Carbon: Fuel (coal, coke), graphite (electrodes, lubricant, pencils), diamond (jewelry, abrasives), activated charcoal (adsorption).
    • Silicon: Semiconductors (transistors, chips), alloys (ferrosilicon), silicones, silica gel.
    • Germanium: Semiconductors (transistors).
    • Tin: Plating (prevent corrosion), alloys (bronze, solder).
    • Lead: Lead storage batteries, alloys (solder), radiation shielding, formerly used in paints and gasoline additives (now restricted due to toxicity).

Multiple Choice Questions (MCQs)

  1. Which of the following Group 13 elements exhibits an atomic radius smaller than the element directly above it?
    (a) Boron
    (b) Aluminium
    (c) Gallium
    (d) Indium

  2. The stability of the +1 oxidation state increases down the Group 13 elements primarily due to:
    (a) Increasing metallic character
    (b) Inert pair effect
    (c) Decreasing ionization enthalpy
    (d) Increasing atomic size

  3. Which of the following boron halides acts as the strongest Lewis acid?
    (a) BF₃
    (b) BCl₃
    (c) BBr₃
    (d) BI₃

  4. The structure of Diborane (B₂H₆) contains:
    (a) Four 2c-2e bonds and two 3c-2e bonds
    (b) Six 2c-2e bonds only
    (c) Two 2c-2e bonds and four 3c-2e bonds
    (d) Four 2c-2e bonds and four B-H-B bridges

  5. Borax bead test is used to identify certain metal ions. The chemical composition of the glassy bead formed is:
    (a) Sodium tetraborate only
    (b) Boric anhydride only
    (c) Sodium metaborate and Boric anhydride
    (d) Sodium orthoborate

  6. Which property is primarily responsible for the unique ability of Carbon to form long chains and rings?
    (a) High electronegativity
    (b) Small atomic size
    (c) High C-C bond enthalpy (Catenation)
    (d) Ability to form pπ–pπ multiple bonds

  7. In the structure of graphite, carbon atoms are hybridized as:
    (a) sp
    (b) sp²
    (c) sp³
    (d) sp³d

  8. Which of the following Group 14 tetrahalides cannot be readily hydrolysed by water?
    (a) CCl₄
    (b) SiCl₄
    (c) GeCl₄
    (d) SnCl₄

  9. Silicones are organosilicon polymers with repeating units. They are known for being:
    (a) Good conductors of electricity
    (b) Highly reactive towards acids
    (c) Water repellents and thermally stable
    (d) Soluble in water

  10. ZSM-5 is an important shape-selective catalyst belonging to the class of:
    (a) Silicones
    (b) Silicates
    (c) Fullerenes
    (d) Zeolites

Answer Key for MCQs:

  1. (c) Gallium (Due to poor shielding of d-electrons)
  2. (b) Inert pair effect
  3. (d) BI₃ (Least back-bonding, most electron deficient Boron)
  4. (a) Four 2c-2e bonds (terminal B-H) and two 3c-2e bonds (bridging B-H-B)
  5. (c) Sodium metaborate and Boric anhydride (NaBO₂ + B₂O₃)
  6. (c) High C-C bond enthalpy (Catenation)
  7. (b) sp² (forming three sigma bonds within the layer, delocalized pi electron)
  8. (a) CCl₄ (Carbon lacks vacant d-orbitals to accept lone pair from water)
  9. (c) Water repellents and thermally stable
  10. (d) Zeolites (Aluminosilicates used as catalysts)

Study these notes thoroughly, focusing on the trends, reasons for anomalies, structures, and key reactions. Understanding the 'why' behind the properties is essential for competitive exams. Good luck!

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