Concept Core
Group 13 to 18 — key properties, allotropes, and anomalous behaviour.
Key Trends Across p-Block (Groups 13–18)
The first member of each group shows anomalous behaviour due to: (1) small atomic size, (2) high electronegativity, (3) absence of d orbitals, (4) high charge/size ratio.
General trends down a group:
Atomic radius ↑ | Ionisation energy ↓ | Metallic character ↑
Electronegativity ↓ | First member: most anomalous
Group 15 — Nitrogen Family
Allotropes of P: White P (toxic, reactive), Red P (stable), Black P (most stable)
Oxoacids of P: H₃PO₄ (orthophosphoric, triprotic), H₃PO₃ (phosphorous, diprotic — one P-H bond, not ionisable), H₃PO₂ (hypophosphorous, monoprotic)
N anomaly: N can form only 3 bonds (no d orbitals), P can expand to 5 (PCl₅, H₃PO₄)
Group 16 — Oxygen Family (Chalcogens)
Allotropes of S: Rhombic (stable at RT), Monoclinic (>96°C)
Oxoacids of S: H₂SO₄ (sulphuric, strong diprotic), H₂SO₃ (sulphurous, weak diprotic), H₂S₂O₇ (oleum/pyrosulphuric), H₂S₂O₈ (peroxodisulphuric — Marshall's acid)
O forms max 2 bonds (no d), S can expand to 6 (SF₆, H₂SO₄).
Group 17 — Halogens
Fluorine anomaly: Most electronegative, no positive oxidation states (no d orbitals), forms only HF (H-bond). Cannot act as oxidising/reducing agent in oxoacid sense.
Oxoacids of Cl: HClO (hypochlorous), HClO₂ (chlorous), HClO₃ (chloric), HClO₄ (perchloric — strongest). Acid strength: HClO₄ > HClO₃ > HClO₂ > HClO
Inter-halogens: ClF, ClF₃, BrF₅, IF₇ (lighter halogen always central)
Group 18 — Noble Gases
Filled outer shells → chemically inert (generally). Xe forms compounds: XeF₂ (linear), XeF₄ (square planar), XeF₆ (distorted octahedral), XeOF₂, XeO₃.
Kr and Xe can form fluorides. He, Ne, Ar: no stable compounds known.
Uses: He (balloons, MRI), Ne (signs), Ar (welding), Kr/Xe (lighting).
Oxoacids — Strength Rules (EAPCET Favourite)
Rule 1: More electronegative central atom → stronger acid (more O-H bond polarisation)
Rule 2: More oxygen atoms on central atom (more terminal oxygens) → stronger acid
Rule 3: P-H bonds in phosphorus oxoacids are NOT acidic (not ionisable)
| Oxoacid | P-OH bonds (acidic) | P-H bonds (not acidic) |
|---|
| H₃PO₄ | 3 | 0 |
| H₃PO₃ | 2 | 1 |
| H₃PO₂ | 1 | 2 |
Formula Vault
Key facts, structures, and rules for p-block elements.
Allotropes of P
White P (P₄) → Red P → Black P
Stability increases left to right
Allotropes of S
Rhombic (stable<96°C)
Monoclinic (>96°C)
Rhombic is stable at room temp
Phosphorus Oxoacids
H₃PO₄ (3H), H₃PO₃ (2H), H₃PO₂ (1H)
P-H bonds not ionisable
Halogen Acid Strength
HF < HCl < HBr < HI
Bond dissociation decreases
Oxyacid Strength
HClO < HClO₂ < HClO₃ < HClO₄
More oxygens = stronger acid
Oxidising Power
F₂ > Cl₂ > Br₂ > I₂
Decreases down Group 17
XeF₂ Shape
Linear (sp³d, 3 lone pairs)
XeF₄: square planar; XeF₆: distorted oct.
Inter-halogens
AB, AB₃, AB₅, AB₇
Lighter halogen is always central
Worked Examples
5 problems — allotropes, oxoacid basicity, structures, oxidising power, and inter-halogens.
EasyHow many P-OH bonds (acidic) does H₃PO₃ have?▾
How many ionisable (acidic) hydrogens does H₃PO₃ have? What is its basicity?
1
H₃PO₃ has 3 H atoms: 2 as P-OH (ionisable) and 1 as P-H (not ionisable).
2
Basicity = number of ionisable H = 2 (diprotic)
✓ H₃PO₃ is diprotic (basicity = 2)
EasyArrange HClO, HClO₂, HClO₃, HClO₄ in increasing order of acid strength▾
Arrange HClO, HClO₂, HClO₃, HClO₄ in increasing acid strength.
1
Rule: more terminal oxygen atoms = more acidic (pulls electron density from O-H bond)
2
HClO: 0 terminal O → weakest. HClO₂: 1. HClO₃: 2. HClO₄: 3 terminal O → strongest.
3
Order: HClO < HClO₂ < HClO₃ < HClO₄
✓ Increasing: HClO < HClO₂ < HClO₃ < HClO₄
MediumWhy can nitrogen not form NF₅ while phosphorus forms PCl₅?▾
Explain why N cannot form NF₅ or NCl₅, whereas P forms PCl₅.
1
Nitrogen: principal quantum number n = 2. Only s and p subshells available in valence shell (2s²2p³). Maximum 4 bonds possible (using one 2s + three 2p = 4 orbitals). No d orbitals in n=2.
2
To form 5 bonds (like PCl₅), an element needs to expand its octet using d orbitals.
3
Phosphorus: n = 3. Has 3d orbitals available → can expand to 5 bonds (PCl₅) or 6 bonds (PCl₆⁻).
4
Result: N maximum valence = 4; P maximum valence = 6.
✓ N has no d orbitals (n=2); P has 3d orbitals allowing valence expansion to 5
EAPCET LevelPredict hybridisation and shape of ClF₃▾
Predict the hybridisation and shape of ClF₃ using VSEPR. Cl is the central atom.
1
Cl: valence electrons = 7. Bonds with 3 F: uses 3 electrons for bonding.
2
Lone pairs on Cl: (7 − 3) / 2 = 2 lone pairs
3
Total electron pairs = 3 (bonding) + 2 (lone) = 5 → sp³d hybridisation
4
VSEPR: 5 pairs in trigonal bipyramidal arrangement. 2 lone pairs occupy equatorial positions (less repulsion). 3 F atoms: 2 axial + 1 equatorial.
5
Shape (atoms only) = T-shaped. Bond angles: 2 axial F-Cl-F not exactly 180° (lone pair compression)
✓ ClF₃: sp³d, T-shaped
Trap QuestionHF is a stronger acid than HCl since F is more electronegative — True?▾
A student argues: 'F is most electronegative, so HF must be the strongest hydrohalic acid.' Evaluate this.
1
The trap: Electronegativity affects O-H bonds in oxyacids but not H-X bonds directly in binary acids.
2
For HX (binary halide acids), acid strength depends on H-X bond dissociation enthalpy and hydration energy.
3
H-F bond is the strongest (very short, very polar) → very hard to break → least ionisation → weakest acid in water.
4
Acid strength: HI > HBr > HCl > HF in aqueous solution.
5
Exception: HF is a weak acid in water despite F being most electronegative.
✓ False — HF is the weakest hydrohalic acid; H-F bond is strongest (highest bond dissociation enthalpy)
Mistake DNA
5 p-block errors that appear in EAPCET every year.
🧪
Counting Wrong Basicity for Phosphorus Oxoacids
Students count all H atoms as acidic. P-H bonds are NOT ionisable.
❌ Wrong
H₃PO₃: 3 H atoms
→ triprotic (basicity 3) ✗
(one P-H bond not acidic)
✓ Correct
H₃PO₃: 2 P-OH bonds ✓
→ diprotic (basicity 2) ✓
H₃PO₂: monoprotic ✓
Only P-OH bonds ionise. P-H bonds are not acidic. Draw the structure: if H is directly on P with no O in between, it's not ionisable.
🌡️
HF Strongest Acid Because F Most Electronegative
In binary halide acids (HX), acid strength increases down the group. Electronegativity logic works for oxyacids, not HX.
❌ Wrong
HF strongest acid (F
most electronegative) ✗
✓ Correct
HF: weakest hydrohalic
HI: strongest ✓
H-F bond: strongest → least
ionised in water
Acid strength of HX in water: HI > HBr > HCl > HF. This is OPPOSITE to what electronegativity alone predicts. Bond strength is the dominant factor here.
🔷
XeF₂ is Angular (Forgetting 3 Lone Pairs on Xe)
Students predict Xe has 1 lone pair (like H₂O structure) for XeF₂, giving angular shape. Actually Xe has 3 lone pairs → linear.
❌ Wrong
XeF₂: bent/angular ✗
(treated like H₂O)
✓ Correct
XeF: 8−2 = 6e⁻ on Xe
= 3 lone pairs ✓
sp³d, linear shape ✓
5 pairs → TBP arrangement
XeF₂: Xe has 3 lone pairs + 2 bond pairs = 5 pairs → sp³d → trigonal bipyramidal e⁻ geometry → lone pairs at equatorial → linear molecular shape.
🔁
Inter-halogens: Wrong Central Atom
In inter-halogen compounds AB_n, A is the heavier halogen (lower in group), NOT the lighter one.
❌ Wrong
In ClF₃: F is central
atom (more electronegative) ✗
✓ Correct
In ClF₃: Cl is central ✓
Lighter halogen (F) is
outer atom ✓
Heavier = central always
In inter-halogens, the heavier (larger) halogen is always the central atom because it can accommodate more bonds and lone pairs using d orbitals.
💡
N₂ is More Stable Than P₂ — Applying Group Trends Incorrectly
The strength of N≡N (triple bond) makes N₂ exceptionally stable. P prefers P₄ with single bonds, not P≡P. This reverses the expected trend.
❌ Wrong
N₂ less stable than P₂
because N is smaller ✗
✓ Correct
N₂: very stable (N≡N, 945
kJ/mol bond energy) ✓
P₂ unstable; P₄ preferred ✓
pπ-pπ bonding in N, not P
N forms strong pπ-pπ multiple bonds due to small size. P's larger size means poor orbital overlap for π bonds; it prefers single bonds in P₄ tetrahedral structure.
Chapter Intelligence
p-Block is the most fact-heavy chapter — a structured approach beats rote memorisation.
EAPCET Weightage (2019–2024)
Oxoacid structures & basicity~9 Group 17 halogen properties~7 Group 15 nitrogen family~6 Noble gas compounds (Xe)~5 Group 16 sulphur allotropes~4 Inter-halogen compounds~3 High-Yield PYQ Patterns
Basicity of H₃PO₃ and H₃PO₂Acid strength order of HClO_xWhy N cannot form NCl₅XeF₂/XeF₄ shape (VSEPR)Allotropes: most stable S/P formOxidising power order of halogensAnomalous behaviour of F vs Cl
Exam Strategy
- Oxoacids of P: memorise basicity by drawing structure. P-OH = acidic. P-H = not acidic. H₃PO₄ (3), H₃PO₃ (2), H₃PO₂ (1).
- Oxyacid strength: more terminal oxygens = more acidic. HClO₄ (3 terminal O) is strongest. HClO (0 terminal O) is weakest.
- Anomalous behaviour: first member of each group is always the odd one out. N vs P, O vs S, F vs Cl — always explained by small size + no d orbitals.
- Noble gas compounds: Xe is the focus. XeF₂ (linear), XeF₄ (square planar), XeF₆ (distorted octahedral). Count lone pairs using valence electrons.
- p-Block links to Chemical Bonding (hybridisation, shapes) and Atomic Structure (electronic configuration explains anomalies).