Concept Core
Group 13 to 18 β key properties, allotropes, and anomalous behaviour.
Key Trends Across p-Block (Groups 13β18)
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 β | Ionisation energy β | Metallic character β
Electronegativity β | First member: most anomalous
Group 15 β Nitrogen Family
Allotropes of P: White P (toxic, reactive), Red P (stable), Black P (most stable)
Oxoacids of P: HβPOβ (orthophosphoric, triprotic), HβPOβ (phosphorous, diprotic β one P-H bond, not ionisable), HβPOβ (hypophosphorous, monoprotic)
N anomaly: N can form only 3 bonds (no d orbitals), P can expand to 5 (PClβ
, HβPOβ)
Group 16 β Oxygen Family (Chalcogens)
Allotropes of S: Rhombic (stable at RT), Monoclinic (>96Β°C)
Oxoacids of S: HβSOβ (sulphuric, strong diprotic), HβSOβ (sulphurous, weak diprotic), HβSβOβ (oleum/pyrosulphuric), HβSβOβ (peroxodisulphuric β Marshall's acid)
O forms max 2 bonds (no d), S can expand to 6 (SFβ, HβSOβ).
Group 17 β 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), HClOβ (chlorous), HClOβ (chloric), HClOβ (perchloric β strongest). Acid strength: HClOβ > HClOβ > HClOβ > HClO
Inter-halogens: ClF, ClFβ, BrFβ
, IFβ (lighter halogen always central)
Group 18 β Noble Gases
Filled outer shells β chemically inert (generally). Xe forms compounds: XeFβ (linear), XeFβ (square planar), XeFβ (distorted octahedral), XeOFβ, XeOβ.
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 β Strength Rules (EAPCET Favourite)
Rule 1: More electronegative central atom β stronger acid (more O-H bond polarisation)
Rule 2: More oxygen atoms on central atom (more terminal oxygens) β 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) |
|---|
| HβPOβ | 3 | 0 |
| HβPOβ | 2 | 1 |
| HβPOβ | 1 | 2 |
Formula Vault
Key facts, structures, and rules for p-block elements.
Allotropes of P
White P (Pβ) β Red P β Black P
Stability increases left to right
Allotropes of S
Rhombic (stable<96Β°C)
Monoclinic (>96Β°C)
Rhombic is stable at room temp
Phosphorus Oxoacids
HβPOβ (3H), HβPOβ (2H), HβPOβ (1H)
P-H bonds not ionisable
Halogen Acid Strength
HF < HCl < HBr < HI
Bond dissociation decreases
Oxyacid Strength
HClO < HClOβ < HClOβ < HClOβ
More oxygens = stronger acid
Oxidising Power
Fβ > Clβ > Brβ > Iβ
Decreases down Group 17
XeFβ Shape
Linear (spΒ³d, 3 lone pairs)
XeFβ: square planar; XeFβ: distorted oct.
Inter-halogens
AB, ABβ, ABβ
, ABβ
Lighter halogen is always central
Worked Examples
5 problems β allotropes, oxoacid basicity, structures, oxidising power, and inter-halogens.
EasyHow many P-OH bonds (acidic) does HβPOβ have?βΎ
How many ionisable (acidic) hydrogens does HβPOβ have? What is its basicity?
1
HβPOβ has 3 H atoms: 2 as P-OH (ionisable) and 1 as P-H (not ionisable).
2
Basicity = number of ionisable H = 2 (diprotic)
β HβPOβ is diprotic (basicity = 2)
EasyArrange HClO, HClOβ, HClOβ, HClOβ in increasing order of acid strengthβΎ
Arrange HClO, HClOβ, HClOβ, HClOβ in increasing acid strength.
1
Rule: more terminal oxygen atoms = more acidic (pulls electron density from O-H bond)
2
HClO: 0 terminal O β weakest. HClOβ: 1. HClOβ: 2. HClOβ: 3 terminal O β strongest.
3
Order: HClO < HClOβ < HClOβ < HClOβ
β Increasing: HClO < HClOβ < HClOβ < HClOβ
MediumWhy can nitrogen not form NFβ
while phosphorus forms PClβ
?βΎ
Explain why N cannot form NFβ
or NClβ
, whereas P forms PClβ
.
1
Nitrogen: principal quantum number n = 2. Only s and p subshells available in valence shell (2sΒ²2pΒ³). Maximum 4 bonds possible (using one 2s + three 2p = 4 orbitals). No d orbitals in n=2.
2
To form 5 bonds (like PClβ
), an element needs to expand its octet using d orbitals.
3
Phosphorus: n = 3. Has 3d orbitals available β can expand to 5 bonds (PClβ
) or 6 bonds (PClββ»).
4
Result: N maximum valence = 4; P maximum valence = 6.
β N has no d orbitals (n=2); P has 3d orbitals allowing valence expansion to 5
EAPCET LevelPredict hybridisation and shape of ClFββΎ
Predict the hybridisation and shape of ClFβ 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 β 3) / 2 = 2 lone pairs
3
Total electron pairs = 3 (bonding) + 2 (lone) = 5 β spΒ³d 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 180Β° (lone pair compression)
β ClFβ: spΒ³d, T-shaped
Trap QuestionHF is a stronger acid than HCl since F is more electronegative β True?βΎ
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) β very hard to break β least ionisation β 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.
β False β 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.
π§ͺ
Counting Wrong Basicity for Phosphorus Oxoacids
Students count all H atoms as acidic. P-H bonds are NOT ionisable.
β Wrong
HβPOβ: 3 H atoms
β triprotic (basicity 3) β
(one P-H bond not acidic)
β Correct
HβPOβ: 2 P-OH bonds β
β diprotic (basicity 2) β
HβPOβ: monoprotic β
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.
π‘οΈ
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.
β Wrong
HF strongest acid (F
most electronegative) β
β Correct
HF: weakest hydrohalic
HI: strongest β
H-F bond: strongest β 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.
π·
XeFβ is Angular (Forgetting 3 Lone Pairs on Xe)
Students predict Xe has 1 lone pair (like HβO structure) for XeFβ, giving angular shape. Actually Xe has 3 lone pairs β linear.
β Wrong
XeFβ: bent/angular β
(treated like HβO)
β Correct
XeF: 8β2 = 6eβ» on Xe
= 3 lone pairs β
spΒ³d, linear shape β
5 pairs β TBP arrangement
XeFβ: Xe has 3 lone pairs + 2 bond pairs = 5 pairs β spΒ³d β trigonal bipyramidal eβ» geometry β lone pairs at equatorial β linear molecular shape.
π
Inter-halogens: Wrong Central Atom
In inter-halogen compounds AB_n, A is the heavier halogen (lower in group), NOT the lighter one.
β Wrong
In ClFβ: F is central
atom (more electronegative) β
β Correct
In ClFβ: Cl is central β
Lighter halogen (F) is
outer atom β
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.
π‘
Nβ is More Stable Than Pβ β Applying Group Trends Incorrectly
The strength of Nβ‘N (triple bond) makes Nβ exceptionally stable. P prefers Pβ with single bonds, not Pβ‘P. This reverses the expected trend.
β Wrong
Nβ less stable than Pβ
because N is smaller β
β Correct
Nβ: very stable (Nβ‘N, 945
kJ/mol bond energy) β
Pβ unstable; Pβ preferred β
pΟ-pΟ bonding in N, not P
N forms strong pΟ-pΟ multiple bonds due to small size. P's larger size means poor orbital overlap for Ο bonds; it prefers single bonds in Pβ tetrahedral structure.
Chapter Intelligence
p-Block is the most fact-heavy chapter β a structured approach beats rote memorisation.
EAPCET Weightage (2019β2024)
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 HβPOβ and HβPOβAcid strength order of HClO_xWhy N cannot form NClβ
XeFβ/XeFβ 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. HβPOβ (3), HβPOβ (2), HβPOβ (1).
- Oxyacid strength: more terminal oxygens = more acidic. HClOβ (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 β always explained by small size + no d orbitals.
- Noble gas compounds: Xe is the focus. XeFβ (linear), XeFβ (square planar), XeFβ (distorted octahedral). Count lone pairs using valence electrons.
- p-Block links to Chemical Bonding (hybridisation, shapes) and Atomic Structure (electronic configuration explains anomalies).
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