p-Block

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Rapid summary for last-minute revision before your exam.

p-Block — Group 13 to 18 Elements (ns²np¹⁻⁶)

The p-block spans Groups 13–18 in the periodic table, containing elements with the last electron entering the p-subshell. These include metals, metalloids, and non-metals, with diverse chemical behavior.

Key Trends (Atomic Radius): Increases down the group as new shells are added. ionization energy decreases down the group. Electronegativity decreases from N to Bi (exception: P > S).

Group 13 (Boron Family): B, Al, Ga, In, Tl

• General electronic configuration: ns²np¹
• Al shows +1 oxidation state (inert pair effect), while B shows +3
• Boron forms covalent compounds (BX₃), Aluminum forms ionic (Al₂O₃)
• Important: Al(OH)₃ is amphoteric — reacts with both acids and bases Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O Al(OH)₃ + NaOH → Na[Al(OH)₄]

Group 14 (Carbon Family): C, Si, Ge, Sn, Pb

• General electronic configuration: ns²np²
• C shows catenation (self-linking) prominently; Si forms π-bonds with O, not with itself
• Sn forms +2 (stannous) and +4 (stannic) oxidation states; Pb shows +2 more stable than +4 (inert pair effect)
• Lead is the only p-block element with no allotrope

Group 15 (Nitrogen Family): N, P, As, Sb, Bi

• General electronic configuration: ns²np³
• All group members show −3, +3, +5 oxidation states (N exceptions: +1, +2, +4)
• N₂ has very high bond dissociation enthalpy (941 kJ/mol) due to triple bond
• P exists as white (P₄, tetrahedral, waxy, reactive) and red (polymeric) allotropes

Group 16 (Oxygen Family): O, S, Se, Te, Po

• General electronic configuration: ns²np⁴
• Oxygen is highly electronegative (3.44), only forms −2 oxidation state in compounds
• S shows +2, +4, +6 oxidation states; S₈ is the most stable allotrope of sulfur
• O₃ (ozone) is a powerful oxidizing agent; O₂ is paramagnetic (triplet)

Group 17 (Halogens): F, Cl, Br, I, At

• General electronic configuration: ns²np⁵
• F₂ is the most electronegative element; smallest atomic size in the period
• HF is a weak acid in water; HCl, HBr, HI are strong acids (HF is weak due to H-bonding and low dissociation)
• Interhalogen compounds: XY, XY₃, XY₅, XY₇ types exist

Group 18 (Noble Gases): He, Ne, Ar, Kr, Xe, Rn

• ns²np⁶ (except He: 1s²)
• Xe forms real compounds: XeF₂, XeF₄ (square planar), XeF₆ (distorted octahedral)
• Clathrate compounds: Noble gases trapped in cavities of other compounds

⚡ Exam tip: Inert pair effect increases down the group — for Tl, +1 is more stable than +3; for Pb, +2 is more stable than +4. This is frequently tested in JEE as “why does PbI₄ not exist?” (Pb(IV) is reduced to Pb(II) by I⁻).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

p-Block — Comprehensive Chemistry Study Guide

Electronic Configuration & Atomic Properties: The p-block elements have the general outer electronic configuration ns²np¹⁻⁶ (except He: 1s²). Atomic size increases down the group. Ionization enthalpy decreases down the group. Electronegativity decreases down group 13 but is irregular in group 16 and 17.

Anomalous Behavior of Second Period: Second period p-block elements (B, C, N, O, F) show anomalous properties due to:

• Small atomic size and high electronegativity
• Absence of d-orbitals in valence shell
• High ionization energy and high lattice/ covalent bond energy
• Examples: Boron forms interstitial compounds and covalent halides; Carbon doesn’t form d-π p-π bonds with itself (unlike Si); Nitrogen has high bond energy due to triple bond; Oxygen doesn’t exhibit +4 oxidation state in fluorides; Fluorine is the most electronegative and reactive element.

Hydrides of p-Block Elements:

Group 13 Hydrides: BH₃ (diborane, B₂H₆) — electron deficient, bridged structure with 3-center-2-electron bonds. B₂H₆ + 2CO → 2BH₃·CO (adduct). AlH₃ is polymeric. GaH₃ is unstable.

Group 14 Hydrides: CH₄ (tetrahedral), SiH₄ (silanes: Si₂H₆, Si₃H₈), GeH₄, SnH₄, PbH₄ (stability decreases down group). SiH₄ is less stable, burns in air, hydrolyzed by alkali.

Group 15 Hydrides: NH₃ (pyramidal, H-bonded, basic), PH₃ (pyramidal, less basic, less soluble), AsH₃, SbH₃, BiH₃ (stability decreases). Bond angles: NH₃ (107.8°) < PH₃ (93.8°). Basicity: NH₃ > PH₃ > AsH₃.

Group 16 Hydrides: H₂O (bent, 104.5°, H-bonded), H₂S, H₂Se, H₂Te, H₂Po (angle decreases down group, property: H₂O has maximum H-bonding, highest boiling point).

Group 17 Hydrides: HF, HCl, HBr, HI — bond energy decreases F > Cl > Br > I. HF is liquid at room temperature (H-bonding), others are gases. Acidity: HF < HCl < HBr < HI.

Oxides of p-Block:

Group 13 Oxides: B₂O₃ (acidic), Al₂O₃ (amphoteric), Ga₂O₃, In₂O₃, Tl₂O₃ (increasingly basic). Al₂O₃ is amphoteric: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]

Group 14 Oxides: CO₂ (linear, acidic), SiO₂ (3D network, acidic), GeO₂, SnO₂, PbO₂. PbO₂ is a strong oxidizing agent: PbO₂ + 4HCl → PbCl₂ + Cl₂ + 2H₂O

Group 15 Oxides: N₂O₃, N₂O₄, N₂O₅, P₄O₆, P₄O₁₀, As₂O₃, Sb₂O₃, Bi₂O₃. NO₂ is brown gas (dimerizes to N₂O₄). P₄O₁₀ is used as a drying agent.

Group 16 Oxides: SO₂ (angular, reducing), SO₃ (planar/trigonal), SeO₂, TeO₂. SO₂ bleaches by reduction: SO₂ + 2H₂O → H₂SO₄ + 2H

Halides of p-Block:

Boron halides: BF₃ (planar, Lewis acid), BCl₃, BBr₃, BI₃. BF₃ is planar sp² with empty p-orbital — strong Lewis acid. It forms adducts: BF₃ + NH₃ → F₃B←NH₃. Hydrolysis: 2BX₃ + 6H₂O → 2B(OH)₃ + 6HX

Silicon halides: SiCl₄ is tetrahedral, hydrolyzes: SiCl₄ + 4H₂O → Si(OH)₄ + 4HCl. SiF₄ is formed from SiO₂ + HF: SiO₂ + 4HF → SiF₄ + 2H₂O; SiF₄ + 2HF → H₂[SiF₆]

Phosphorus halides: PCl₃ (pyramidal) and PCl₅ (trigonal bipyramidal). PCl₅ in solid exists as [PCl₄]⁺[PCl₆]⁻. Hydrolysis: PCl₃ + 3H₂O → H₃PO₃ + 3HCl; PCl₅ + 4H₂O → H₃PO₄ + 5HCl

Sulfur halides: SF₆ (octahedral, very stable), S₂Cl₂, SCl₂, SF₄ (see-saw), S₂F₁₀. SF₆ is extremely inert due to steric hindrance.

Pseudohalogens: Compounds that behave like halogens: (CN)₂, (SCN)₂, (OCN)₂. Their ions: CN⁻, SCN⁻, OCN⁻. Interhalogen compounds: ClF, BrF₃, ICl₃, IF₅, IF₇.

⚡ Exam tip: When asked about hydrolysis of p-block halides — product depends on the element’s oxidation state and the halide’s nature. PCl₃ gives H₃PO₃ (phosphorous acid), PCl₅ gives H₃PO₄ (phosphoric acid). SiCl₄ gives SiO₂·xH₂O (silicic acid gel). NCl₃ gives NH₃ + HOCl (different mechanism).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

p-Block — Comprehensive Chemistry Notes

Diborane (B₂H₆) — Structure and Reactions:

Diborane has a unique bridge structure with two boron atoms and four bridging hydrogen atoms. The bridge bonds are 3-center-2-electron bonds (B—H—B), where two electrons are shared among three atoms. Each boron is sp³ hybridized. Four terminal B—H bonds are normal 2-center-2-electron bonds.

        H   H
        |   |
    H—B---B—H
   / \ / \ 
  H   H   H

Key reactions of diborane:

1. With water: B₂H₆ + 6H₂O → 2B(OH)₃ + 6H₂
2. With ammonia: B₂H₆ + 2NH₃ → 2BH₃·NH₃ (adduct) → at higher T → B₃N₃H₆ (inorganic benzene analog)
3. With CO: B₂H₆ + 2CO → 2BH₃·CO
4. Combustion: B₂H₆ + 3O₂ → B₂O₃ + 3H₂O (highly exothermic)

Metalloids and Their Properties: Boron and silicon are metalloids with high melting points, brittleness, and semiconductor properties. Boron is extremely hard (second only to diamond in hardness among elements).

Compounds with π-Bonds (pπ-pπ Backbonding):

• BF₃ can accept electron density via empty p-orbital (Lewis acid)
• BF₃ back-donates electron density to empty p-orbital on B, forming pπ-pπ bonding
• This makes B—F bond shorter than expected and gives partial double bond character
• Bond length: BF < BCl < BBr < BI

Oxoacids of Phosphorus:

1. Hypophosphorous acid (H₃PO₂): One P—H bond, one P=O, one P—OH. Also called phosphinic acid. It’s a reducing agent (monobasic). Structure: O=P(OH)H₂

2. Phosphorous acid (H₃PO₃): Two P—OH, one P—H, one P=O. Dibasic. Structure: O=P(OH)₂H

3. Phosphoric acid (H₃PO₄): Three P—OH, one P=O. Tribasic, tribasic, not an oxidizing agent. Structure: O=P(OH)₃

4. Pyrophosphoric acid (H₄P₂O₇): Two P atoms connected by P—O—P bond. Structure: (HO)₂P(=O)—O—P(=O)(OH)₂

5. Metaphosphoric acid (HPO₃): Cyclic trimer (HPO₃)₃ or linear polymer.

Sulfuric Acid (H₂SO₄) — King of Acids:

• Non-volatile acid (used to prepare other volatile acids like HCl, HNO₃)
• Strong dehydrating agent: C₁₂H₂₂O₁₁ + H₂SO₄ → 12C + 11H₂O
• Oxidizing agent: H₂SO₄ (hot, conc.) + 2HI → I₂ + SO₂ + 2H₂O
• SO₃ is formed from SO₂ + ½O₂ (contact process): 2SO₂ + O₂ ⇌ 2SO₃ (catalyzed by V₂O₅)

Nitric Acid (HNO₃) — Preparation and Reactions: Ostwald’s process: 4NH₃ + 5O₂ → 4NO + 6H₂O (Pt/Rh catalyst at 900°C); 2NO + O₂ → 2NO₂; 3NO₂ + H₂O → 2HNO₃ + NO

Properties:

• Concentrated HNO₃ is 68% by weight, 15 M
• Decomposes: 4HNO₃ → 4NO₂ + 2H₂O + O₂
• Passivation: Fe, Al, Cr become passive due to oxide layer
• Nitration: C₆H₆ + HNO₃ (conc.) → C₆H₅NO₂ + H₂O (requires conc. H₂SO₄ as catalyst)

XeF₆ Hydrolysis: XeF₆ + H₂O → XeOF₄ + 2HF XeF₆ + 3H₂O → XeO₃ + 6HF XeF₆ is a strong fluorinating agent.

Interhalogen Compounds — Shapes from VSEPR:

• XY (ClF, BrCl): Linear
• XY₃ (ClF₃): T-shaped (2 lone pairs on central atom)
• XY₅ (BrF₅): Square pyramidal (1 lone pair)
• XY₇ (IF₇): Pentagonal bipyramidal

Silicates: Silicates are salts of silicic acid (H₄SiO₄). Classification:

• Orthosilicates: [SiO₄]⁴⁻ (e.g., olivine, Mg₂SiO₄)
• Pyrosilicates: [Si₂O₇]⁶⁻ (two tetrahedra sharing one oxygen)
• Cyclic silicates: [Si₃O₉]⁶⁻, [Si₄O₁₂]⁸⁻
• Chain silicates: [SiO₃]²⁻ (single chain) or [Si₄O₁₁]⁶⁻ (double chain)
• Sheet silicates: [Si₂O₅]²⁻ (2D network)
• Three-dimensional silicates: [AlSi₃O₁₀]ⁿ⁻ (aluminosilicates, e.g., feldspars, zeolites)

Zeolites are porous aluminosilicates used as catalysts and water softeners.

Fullerenes and Carbon Nanostructures: C₆₀ (buckminsterfullerene) has 20 hexagonal and 12 pentagonal faces, with sp² hybridized carbons. C₇₀ is an elongated cage. These are molecular carbon allotropes distinct from diamond (sp³) and graphite (sp²).

Comparative Study — Hydrides:

Property	NH₃	PH₃
Bond angle	107.8°	93.8°
Basicity	Strong base	Weak base
Boiling point	−33°C	−87.5°C
Reducing character	Weak	Strong

Property	H₂O	H₂S	H₂Se	H₂Te
Bond angle	104.5°	92.1°	91°	90°
Acidic character	Neutral	Weak acid	Acidic	More acidic
Boiling point	100°C	−60°C	−41°C	−2°C

Environmental Significance:

• SO₂ causes acid rain: SO₂ + H₂O → H₂SO₃ → 2H⁺ + SO₃²⁻
• NOₓ causes photochemical smog
• CFCs destroy ozone layer: CFCl₃ + UV → Cl· + ·CFCl₂; Cl· + O₃ → ClO· + O₂
• CO is toxic (binds to hemoglobin); CO₂ causes greenhouse effect

Biological Significance:

• Boron is essential for plant cell wall integrity
• Silicon is essential for diatoms and some plants
• Arsenic is toxic (replaces phosphorus in enzymes)
• Nitrogen is essential (proteins, DNA)

⚡ Exam tip: When comparing acidic strength of oxoacids with the same central atom in different oxidation states — the acid with the higher oxidation state is stronger (more electron-withdrawing oxygen atoms). Thus: H₂SO₄ > H₂SO₃ > H₂S₂O₃; H₃PO₄ > H₃PO₃ > H₃PO₂. But when comparing acids with different central atoms in the same group — the one with the more electronegative central atom is stronger (HF > HCl > HBr > HI in terms of acidity of binary hydrides).

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