p-Block Elements

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

p-Block Elements — Key Facts for CUET Group 13 (Boron family): $ns^2 np^1$; Al → amphoteric oxide; In and Tl show +1 oxidation state (inert pair effect) Group 14 (Carbon family): $ns^2 np^2$; Sn and Pb form +2 and +4 states; Pb(IV) is oxidising (PbO₂) Group 15 (Nitrogen family): $ns^2 np^3$; anomalous properties of N; allotropes: N₂ (triple bond, inert), P (white/red), As, Sb Group 16 (Oxygen family): $ns^2 np^4$; O₂ is paramagnetic (triplet); ozone O₃; allotropes of S: S₈ (crown), plastic S Group 17 (Halogens): $ns^2 np^5$; most electronegative group; F₂ is strongest oxidising agent; HF is weak acid Group 18 (Noble gases): $ns^2 np^6$; He = 0.0005% of atmosphere; Xe forms compounds (XeF₂, XeF₄, XeF₆, XeO₃) ⚡ Exam tip: Inert pair effect increases down the group — heavier elements prefer lower oxidation state (+2 for Sn, Pb; +1 for Tl)

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding of p-block chemistry trends.

p-Block Elements — CUET Chemistry Study Guide

The p-block spans groups 13–18, containing a remarkable diversity of elements from metals (Sn, Pb, Bi) to non-metals (C, N, O, F, Ne) to metalloids (B, Si, Ge, As, Sb, Te). The general electronic configuration is $ns^2np^{1-6}$.

Group 13 — Boron Family:

• Boron (B): $1s^2 2s^2 2p^1$, $sp^2$ hybridised, forms covalent compounds (BX₃). Boric acid B(OH)₃ is a weak monobasic acid (Lewis acid: accepts OH⁻).
• Aluminium (Al): most abundant metal in Earth’s crust (8.3%). Amphoteric — reacts with acids AND bases: $2Al + 6HCl → 2AlCl₃ + 3H₂$; $2Al + 2NaOH + 6H₂O → 2Na[Al(OH)₄] + 3H₂$.
• Inert pair effect: reluctance of $s$-electrons to participate in bonding, increasing down the group (B < Al < Ga < In < Tl). This makes lower oxidation states increasingly stable down the group.

Group 14 — Carbon Family:

• Carbon (C): unique due to catenation, forms $sp$, $sp^2$, $sp^3$ hybridisation. Allotropes: diamond ($sp^3$, tetrahedral, hard), graphite ($sp^2$, layered, lubricating), fullerenes ($sp^2$, spherical C₆₀).
• Silicon (Si): second most abundant element in Earth’s crust. $sp^3$ hybridised, forms covalent network solids. SiO₂ (quartz, sand) — each Si tetrahedrally bonded to 4 O atoms.
• Tin (Sn): two allotropes — grey Sn (diamond structure, $<13.2°$C) and white Sn (metallic, $>13.2°$C). Sn forms SnCl₂ (reducing agent) and SnCl₄ (Lewis acid).
• Lead (Pb): final stable element. Pb(II) is more stable than Pb(IV). PbO₂ is a strong oxidising agent (in lead-acid battery).

Group 15 — Nitrogen Family:

• Nitrogen: N₂ has triple bond (bond energy 945 kJ/mol), making it chemically inert at room temperature. High electronegativity (3.0). Forms hydrides (NH₃, pKₐ 9.25), oxides (NO, NO₂, N₂O₃, N₂O₄, N₂O₅), oxyacids (HNO₂, HNO₃).
• Phosphorus: Allotropes — white P (P₄, tetrahedral, reactive, toxic, glows in dark) and red P (polymeric, more stable). PCl₃ and PCl₅ are important compounds.
• Oxoacids of phosphorus: Hypophosphorous (H₃PO₂ — one ionisable H), Phosphorous (H₃PO₃ — two ionisable H), Phosphoric (H₃PO₄ — three ionisable H, tribasic).

Group 16 — Oxygen Family:

• Oxygen: O₂ is paramagnetic (triplet diradical, 2 unpaired electrons). Ozone O₃ is an allotrope — bent molecule, stronger oxidising than O₂.
• Sulphur: Allotropes — S₈ crown-shaped (most stable at room temperature), plastic S (amorphous, quenched from melt). SO₂ (pyramidal) and SO₃ (planar in gas, cyclic trimer in solid).

Example: Why is BF₃ a Lewis acid but NH₃ is a Lewis base? BF₃ has an incomplete octet (B is $sp^2$, only 6 electrons around B) — it can accept an electron pair from a donor like NH₃. NH₃ has a lone pair on N that can be donated. The reaction BF₃ + NH₃ → H₃N-BF₃ forms an adduct.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

p-Block Elements — Complete CUET Chemistry Notes

Allotropes of Carbon — Deep Dive:

• Diamond: $sp^3$ hybridised, tetrahedral network, C-C bond length 154 pm, bond energy 347 kJ/mol. Hardest natural substance. Band gap 5.5 eV (insulator).
• Graphite: $sp^2$ hybridised, layered structure (hexagonal sheets), layers held by van der Waals forces (weak). Electrons delocalised in π system → conducts electricity. Used as lubricant and electrode.
• Graphene: single layer of graphite, $sp^2$ carbons in hexagonal lattice. Extraordinary properties: Young’s modulus ~1 TPa, electron mobility 200,000 cm²/Vs. Used in flexible electronics, composites.
• Fullerenes (C₆₀, C₇₀): truncated icosahedron. C₆₀ has 12 pentagons + 20 hexagons. Endohedral fullerenes (atoms inside cage) used in drug delivery.
• Carbon nanotubes: rolled-up graphene sheets. Single-walled (SWNT) and multi-walled (MWNT). Remarkable tensile strength, used in nanotechnology.

Interhalogen Compounds: Formed between halogens (XX’). Types: XY (ClF, BrF, ICl), XY₃ (ClF₃, BrF₃, ICl₃), XY₅ (IF₅, BrF₅), XY₇ (IF₇, largest interhalogen). More electronegative halogen is central atom. Used as fluorinating agents.

Pseudohalogens: Groups that behave like halogens: CN⁻ (cyanide), SCN⁻ (thiocyanate). (CN)₂ is cyanogen — toxic gas. Thionyl chloride SOCl₂ — used to convert alcohols to chlorides.

Noble Gas Compounds:

• XeF₂: linear, $sp^3d$ hybridised (bheteronuclear), oxidising agent
• XeF₄: square planar, $sp^3d^2$ hybridised
• XeF₆: distorted octahedral, $sp^3d^3$ hybridised
• XeO₃: xenon trioxide, explosive, trigonal pyramidal
• Clathrates: noble gases trapped in host crystal lattices (used to separate Kr from Xe in air)

Heterocyclic Silicon Chemistry: Unlike carbon, silicon rarely forms stable double bonds (pπ-pπ bonds are weak). However, compounds with Si=Si double bonds (disilenes) can be isolated if bulky groups prevent polymerisation. Silicones: (R₂SiO)ₙ — alternating Si-O backbone, organic groups on Si. Used as sealants, lubricants.

Boron Hydrides and Wade’s Rules: Boron forms electron-deficient cluster compounds. BH₃ (monomer) doesn’t exist as stable — dimerises to B₂H₆ (diborane). Structure: bridging H atoms held by 3c-2e bonds. Wade’s rules: closo (BₙHₙ²⁻), nido (BₙHₙ₊₄), arachno (BₙHₙ₊₆) cluster structures based on skeletal electron pairs.

CUET Exam Patterns (2022–2024):

• Properties of p-block elements and trends down the group are most frequently tested
• Allotropes of phosphorus (white/red) and carbon (diamond/graphite) are common MCQs
• Inert pair effect and variable oxidation states appear every year
• Interhalogen and noble gas compounds occasionally tested
• Common mistakes: confusing oxidation states; not remembering amphoteric nature of Al and Zn; forgetting that F is always -1 in compounds (except with O)

⚡ Key insight: In p-block, electronegativity and oxidation state are key. Elements become less electronegative down the group (F is most electronegative element ever). The inert pair effect makes +2 oxidation state more stable down group 14 (Sn(II) ≠ unstable like C(IV); Pb(II) more stable than Pb(IV)). Remember that the most electronegative elements (F, O) always have negative oxidation states in compounds.

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