The p-Block Elements — Groups 15 to 18 · NEET Study Pack

🗓️How to Study — Your 5-Day Plan

This is the single densest chapter in inorganic chemistry. Trying to memorise everything in one pass never works. The trick: study one group per day, then compare across on Day 5. Comparing trends is what NEET actually tests.

Day 1 · 2.5 hrs

Group 15 — Nitrogen Family

Trends + NH₃ (Haber) + oxides of N + HNO₃ (Ostwald + brown-ring test) + phosphorus allotropes + PCl₃/PCl₅ + H₃PO₃ dibasic trap.

Day 2 · 2 hrs

Group 16 — Oxygen Family

Trends + ozone structure & bent shape + sulphur allotropes + SO₂ + H₂SO₄ (Contact process, oxidising vs dehydrating) + oxoacids.

Day 3 · 2 hrs

Group 17 — Halogens

Trends + HF anomaly + bond dissociation (Cl > Br > F > I) + oxoacids of Cl (acidity order) + interhalogens (XX', XX'₃, XX'₅, XX'₇) + bleaching powder.

Day 4 · 1.5 hrs

Group 18 — Noble Gases

Xenon compounds: XeF₂ linear · XeF₄ square planar · XeF₆ distorted octahedral + XeO₃ + XeOF₄ hybridisation table. Small block, high yield.

Day 5 · 2 hrs

Cross-group + chapter test

Compare trends across all 4 groups on one A4. Sit a 45-Q chapter test cold. Log errors by group so you know where to repair.

📌 The one rule that makes this chapter feel manageable

Learn each group as a "top-down story". For every group, first fix the trend direction (usually decreases down for EN, IE, EA · increases for atomic size, metallic character), then note the anomaly of the first element (N, O, F all break patterns), then learn 3-4 key compounds. Don't jump between groups.

📚Topic Map — 4 Groups, 16 Blocks

Every NEET question comes from one of these 16 blocks. 🔥 count = typical frequency across recent NEET papers.

G15 · Block 1

Group 15 trends + anomalous N

ns²np³ config, oxidation states −3 to +5, N's lack of d-orbitals ⇒ max covalency = 4 (NCl₃ but not NCl₅).

🔥🔥 · 1 Q/yr

G15 · Block 2

Ammonia (NH₃) & Haber process

Trigonal pyramidal, sp³, H-bonding. Haber: N₂+3H₂ → 2NH₃, Fe catalyst + Mo promoter, 200 atm, 673 K.

🔥🔥🔥 · 1 Q/yr

G15 · Block 3

Oxides of nitrogen + HNO₃

N₂O, NO, N₂O₃, NO₂, N₂O₄, N₂O₅. Ostwald process, brown-ring test, dilute vs conc HNO₃ oxidises metals differently.

🔥🔥🔥 · 1 Q/yr

G15 · Block 4

Phosphorus — allotropes + halides + oxoacids

White (P₄, reactive), red (polymeric, safe), black (most stable). PCl₃ pyramidal · PCl₅ gas TBP / solid ionic [PCl₄]⁺[PCl₆]⁻. H₃PO₃ is dibasic (only 2 P-OH), H₃PO₄ is tribasic.

🔥🔥🔥🔥 · 1-2 Q/yr

G16 · Block 5

Group 16 trends + anomalous O

ns²np⁴. O has no d-orbitals ⇒ max covalency 2; S can expand to 6 (SF₆).

🔥🔥 · 1 Q every 2 yrs

G16 · Block 6

Ozone (O₃)

Bent, 117°, resonance hybrid. Absorbs UV in stratosphere. Powerful oxidiser.

🔥🔥 · 1 Q every 2 yrs

G16 · Block 7

Sulphur allotropes + SO₂

Rhombic α-S (S₈, crown), monoclinic β-S. SO₂ bent (V-shaped), sp², bleaches by reduction (unlike Cl₂).

🔥🔥 · 1 Q every 2 yrs

G16 · Block 8

H₂SO₄ — Contact process + reactions

S+O₂→SO₂ →(V₂O₅, 450°C) SO₃ →(oleum → H₂SO₄). King of chemicals. Oxidising (with Cu), dehydrating (sugar → C).

🔥🔥🔥 · 1 Q/yr

G17 · Block 9

Halogen trends + F anomaly

Highest EN group. Bond dissoc: Cl₂ > Br₂ > F₂ > I₂ (F₂ anomaly: lone-pair repulsion in small F-F). EN: F > Cl > Br > I.

🔥🔥🔥 · 1-2 Q/yr

G17 · Block 10

Hydrohalic acids (HX)

Acid strength: HF < HCl < HBr < HI (bond strength decreases down). But HF is anomalous: liquid at room temp, H-bonding.

🔥🔥🔥 · 1 Q/yr

G17 · Block 11

Oxoacids of Cl + bleaching powder

HOCl < HClO₂ < HClO₃ < HClO₄ (acidity + oxidation state increase together). Bleaching powder Ca(OCl)Cl.

🔥🔥🔥 · 1 Q/yr

G17 · Block 12

Interhalogens + pseudohalogens

4 types by CN: XX' (ClF, BrCl), XX'₃ (ClF₃ T-shape), XX'₅ (BrF₅ square pyramid), XX'₇ (IF₇ pentagonal bipyramid). Pseudohalogens: (CN)₂, (SCN)₂.

🔥🔥🔥🔥 · 1-2 Q/yr

G18 · Block 13

Noble gases — general

Closed shell ns²np⁶ (except He 1s²). Chemically inert till 1962. Uses: He (MRI, balloons), Ne (lights), Ar (bulbs).

🔥 · every 2-3 yrs

G18 · Block 14

Xenon fluorides (XeF₂/₄/₆)

XeF₂ linear (sp³d), XeF₄ square planar (sp³d²), XeF₆ distorted octahedral (sp³d³). Structure-vs-hybridisation table is guaranteed marks.

🔥🔥🔥🔥 · 1-2 Q/yr

G18 · Block 15

Xenon oxides + oxyfluorides

XeO₃ (pyramidal), XeO₄ (tetrahedral), XeOF₄ (square pyramidal), XeO₂F₂ (see-saw).

🔥🔥 · every 2 yrs

Cross · Block 16

Cross-group comparisons

Anomalous first elements (N, O, F). Down-group trend directions. Diagonal relationships.

🔥🔥🔥 · 1 Q/yr

15Group 15 — Nitrogen Family (N · P · As · Sb · Bi)

Concept 1 · trends

Group trends down the family

Electronic configuration: ns² np³ · half-filled p-subshell → extra stability, higher IE.
Atomic radius increases: N < P < As < Sb < Bi.
Ionisation enthalpy decreases down. But N > O (across period) due to half-filled p stability.
Electronegativity decreases down; N (3.0) most electronegative.
Metallic character increases down: N/P non-metals · As/Sb metalloids · Bi metal.
Oxidation states: −3, +3, +5 most common. Down the group, +3 becomes more stable (inert pair effect — Bi mostly +3).

Concept 2 · anomaly of N

Why nitrogen behaves differently

No d-orbitals in valence shell → maximum covalency of 4 (unlike P which reaches 5 or 6). Hence NCl₃ exists, NCl₅ doesn't; PCl₅ exists.
Small size + high EN + no d-orbitals → forms strong pπ-pπ multiple bonds (N≡N in N₂). Other Group 15 elements prefer catenated single bonds instead.
N₂ has very high bond dissociation energy (945 kJ/mol) due to the triple bond → makes N₂ almost inert at room temp.
Cannot form dπ-pπ bonds (no d-orbitals); P readily can (e.g. P=O in H₃PO₄).

Concept 3 · NH₃ + Haber

Ammonia — structure, prep, uses

Structure: trigonal pyramidal (sp³), one lone pair. Bond angle 107° (less than 109.5° due to lp-bp repulsion).
Haber process: N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92 kJ/mol. Conditions: Fe catalyst + Mo promoter, ~200 atm, ~700 K. Le Chatelier says high P + low T favour product; kinetics forces a compromise.
Basicity: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃ (lone-pair availability decreases as size increases).
H-bonding: NH₃ shows strong H-bonding (N is small, high EN) → high boiling point (−33 °C) vs PH₃ (−88 °C).

Concept 4 · oxides + HNO₃

Nitrogen oxides + nitric acid

Oxide	N oxidation state	Property
N₂O (laughing gas)	+1	Neutral · anaesthetic · linear
NO (nitric oxide)	+2	Neutral · odd-electron molecule · paramagnetic
N₂O₃	+3	Acidic · anhydride of HNO₂
NO₂	+4	Brown · paramagnetic · dimerises to N₂O₄
N₂O₅	+5	Acidic · anhydride of HNO₃

HNO₃ — Ostwald process: 4NH₃ + 5O₂ →(Pt/Rh, 500°C) 4NO + 6H₂O → NO₂ → HNO₃. Products depend on concentration: dilute HNO₃ + dilute-metal → gives H₂? No — HNO₃ never gives H₂ because NO₃⁻ oxidises H⁻ to H₂O. Products: NO, N₂O, NH₄NO₃ depending on conditions.

Brown ring test (for NO₃⁻): salt + FeSO₄ + conc. H₂SO₄ (dropped slowly) → forms [Fe(H₂O)₅NO]²⁺ brown ring at interface. Diagnostic for nitrates.

Concept 5 · phosphorus

Phosphorus — allotropes, halides, oxoacids

Allotropes:

White phosphorus — P₄ tetrahedral units, highly reactive, glows in dark, stored under water, toxic.
Red phosphorus — polymeric chains of P₄, unreactive at room temp, non-toxic, safer.
Black phosphorus — most thermodynamically stable, semiconductor.

Halides:

PCl₃ — pyramidal (sp³, lone pair). Hydrolyses to H₃PO₃ + HCl.
PCl₅ — trigonal bipyramidal (sp³d) in gas · ionic [PCl₄]⁺[PCl₆]⁻ in solid state (tetrahedral cation + octahedral anion — classic NEET trap).

Oxoacids of phosphorus:

Acid	Formula	Basicity	P-H bonds
Hypophosphorous	H₃PO₂	Mono	2
Phosphorous (phosphonic)	H₃PO₃	Di · NEET trap	1
Orthophosphoric	H₃PO₄	Tri	0
Pyrophosphoric	H₄P₂O₇	Tetra	0

Rule: basicity = number of O-H bonds only. P-H bonds do NOT ionise. H₃PO₃ has 2 O-H and 1 P-H → dibasic (H's on P don't count).

16Group 16 — Oxygen Family (O · S · Se · Te · Po)

Concept 6 · trends

Group 16 trends

Config: ns² np⁴ · two electrons short of noble gas → typically −2 or +2/+4/+6.
Atomic radius increases down; O is very small (66 pm).
EN: O > S > Se > Te > Po. O is 2nd-most-electronegative element (only F higher).
Metallic character: O/S non-metals · Se/Te metalloids · Po metal.
Oxidation states: O usually −2 (fixed, no d-orbitals). S: −2, +2, +4, +6. Down, +4 more stable than +6.

Concept 7 · anomaly of O

Oxygen anomaly

No d-orbitals → max covalency 2 (H₂O, not H₂O₄). S extends to 6 (SF₆).
Small + high EN → strong pπ-pπ multiple bonds (O=O in O₂). S prefers catenated single bonds (S₈ ring).
Explains why O₂ is a gas (small discrete molecules with π-bond) while S₈ is a solid (cyclic single-bonded ring).

Concept 8 · ozone

Ozone (O₃)

Structure: bent · O-O-O angle = 117° · both O-O bonds equal (1.28 Å) — intermediate between O-O single (1.48) and O=O double (1.21). Resonance hybrid of two forms.
Uses: disinfectant, bleach, protective UV shield in stratosphere.
Powerful oxidiser: releases nascent O; commonly written as O₃ → O₂ + [O]. Oxidises KI to I₂ (starch-iodide paper test).

Concept 9 · sulphur

Sulphur allotropes + SO₂

Rhombic (α-S): S₈ crown-shaped rings, stable below 96 °C, yellow, most stable at room temp.
Monoclinic (β-S): also S₈ rings, stable 96-119 °C, needle-shaped.
Both transition at 96 °C (transition temperature).
Plastic sulphur: long S-chain polymer, unstable.

SO₂: bent (V-shape), sp², bond angle 119°. Bleaches by reduction (via nascent H, temporary — colour returns) — differs from Cl₂ which bleaches by oxidation, permanent. Classic NEET distinction.

Concept 10 · H₂SO₄

Sulphuric acid — "king of chemicals"

Contact Process:

S + O₂ → SO₂
2SO₂ + O₂ ⇌ 2SO₃ (V₂O₅ catalyst, 450 °C, 1-2 atm)
SO₃ + H₂SO₄ → H₂S₂O₇ (oleum). Then H₂S₂O₇ + H₂O → 2H₂SO₄. (Direct absorption of SO₃ in water is too exothermic — forms mist.)

Three natures:

Strong acid: Ka₁ >> 1 (fully ionised in first step). Ka₂ ≈ 1.2×10⁻².
Oxidising: hot conc. H₂SO₄ oxidises Cu → CuSO₄ + SO₂ + H₂O. Oxidises C, S, HBr, HI.
Dehydrating: removes water from sugars, HCOOH → CO. Blackens sugar into carbon.

17Group 17 — Halogens (F · Cl · Br · I · At)

Concept 11 · trends

Halogen family trends

Config: ns² np⁵ · one short of noble gas → highly electronegative, gain e⁻ readily.
EN: F (4.0) > Cl (3.2) > Br (3.0) > I (2.7). F is most electronegative element.
Electron affinity: Cl > F > Br > I. F anomaly — small size causes electron-electron repulsion when new e⁻ enters compact 2p.
Bond dissociation energy: Cl₂ (243) > Br₂ (192) > F₂ (159) > I₂ (151) kJ/mol. F₂ anomaly — small F-F distance → strong lone-pair repulsion between the two F atoms. This is why F₂ is more reactive than expected.
Oxidising power: F₂ > Cl₂ > Br₂ > I₂ (F₂ is the strongest oxidiser known).

Concept 12 · HX

Hydrohalic acids (HX)

Acid strength: HF (weakest) < HCl < HBr < HI (strongest). Reason: H-X bond gets weaker down the group → easier to ionise.
HF anomaly: despite F being most EN, HF is the weakest acid. Also HF is liquid at room temp (BP 20 °C), all others gases. Reason: strong H-F H-bonding gives long chains (HF)_n, needing extra energy to dissociate.
Bond dissociation energy of HX: HF (568) > HCl (432) > HBr (366) > HI (298) kJ/mol. Consistent with acid strength trend.
Thermal stability: HF > HCl > HBr > HI. HI decomposes on heating; HF is very stable.

Concept 13 · oxoacids of Cl

Oxoacids of chlorine — acidity + oxidation state ladder

Oxoacid	Formula	Cl oxidation state	Acid strength
Hypochlorous	HOCl (or HClO)	+1	Weakest
Chlorous	HClO₂	+3	↑
Chloric	HClO₃	+5	↑
Perchloric	HClO₄	+7	Strongest (super acid)

Rule: as oxidation state increases, more O atoms delocalise negative charge on the conjugate anion → conjugate base more stable → acid stronger.

Bleaching powder: Ca(OCl)Cl · 3H₂O. Prep: 2Ca(OH)₂ + 2Cl₂ → Ca(OCl)₂ + CaCl₂ + 2H₂O (or as mixed salt). Releases Cl₂ on treatment with dilute acid — active bleach.

Concept 14 · interhalogens

Interhalogens — 4 types by coordination

Type	Shape	Hybridisation	Examples
XX' · CN 1	Linear	—	ClF, BrF, BrCl, ICl
XX'₃ · CN 3	T-shaped (2 lp)	sp³d	ClF₃, BrF₃, IF₃
XX'₅ · CN 5	Square pyramidal (1 lp)	sp³d²	BrF₅, IF₅
XX'₇ · CN 7	Pentagonal bipyramidal (0 lp)	sp³d³	IF₇ only

General rule: the larger halogen sits in the centre; the smaller/more EN halogen is the substituent. Interhalogens are more reactive than the parent halogens because X-X' bonds are weaker than X-X or X'-X' bonds.

Pseudohalogens: molecules with X₂-like behaviour: (CN)₂ cyanogen, (SCN)₂ thiocyanogen, (OCN)₂. Form pseudohalides like CN⁻, SCN⁻, OCN⁻.

18Group 18 — Noble Gases (He · Ne · Ar · Kr · Xe · Rn)

Concept 15 · overview

Why noble gases are (mostly) inert

Config: ns² np⁶ · except He (1s²). Fully filled valence shell → extreme stability.
Very high ionisation enthalpy: He (2372), Ne (2080), Ar (1520) kJ/mol — reluctant to give up e⁻.
Positive electron gain enthalpy: they don't want to accept e⁻ either.
All are monoatomic gases at room temp. Weak dispersion forces only → very low BP.
Chemistry started in 1962 when Neil Bartlett made Xe⁺[PtF₆]⁻ from Xe + PtF₆. Xe is the only noble gas with rich chemistry (large size → lower IE → can lose e⁻).

Concept 16 · xenon fluorides

Xenon fluorides — structure, hybridisation, geometry

Compound	Prep	Hybridisation	Shape	Lone pairs on Xe
XeF₂	Xe + F₂ (1:2, 673 K)	sp³d	Linear	3
XeF₄	Xe + F₂ (1:5, 873 K)	sp³d²	Square planar	2
XeF₆	Xe + F₂ (1:20, 573 K, 60 atm)	sp³d³	Distorted octahedral (VSEPR: 6 bp + 1 lp)	1

Bonding intuition: Xe uses its 5s + 5p + 5d orbitals to form σ bonds to F. F pulls electron density strongly (F is most EN); Xe accommodates.

Concept 17 · xenon oxides + oxyfluorides

Xenon oxides and oxyfluorides

Compound	Shape	Hybridisation
XeO₃	Pyramidal (3 bp + 1 lp)	sp³
XeO₄	Tetrahedral	sp³
XeOF₂	T-shaped	sp³d
XeOF₄	Square pyramidal	sp³d²
XeO₂F₂	See-saw (2 bp O + 2 bp F + 1 lp)	sp³d

Uses of noble gases: He — MRI, balloons, deep-sea diving (Heliox); Ne — advertising signs; Ar — inert atmosphere in bulbs and welding; Kr — flash lamps; Xe — arc lamps, anaesthesia.

🎯 p-Block Revision Pack — Groups 15, 16, 17, 18

📅 Built: 2026-07-20 Target: Top-score Inorganic

Dense, exam-only revision for the four p-Block groups you must know cold. 🔥 flags the highest-yield facts NEET repeats every year. Strict-NCERT throughout; any beyond-NCERT item is rose-tagged. All equations balanced; all bond angles, hybridisations and oxidation states exact.

1 Periodic Trends Down the Group Always tested

Property	Group 15 (N→Bi)	Group 16 (O→Po)	Group 17 (F→At)	Group 18 (He→Rn)
Atomic / ionic radius	Increases	Increases	Increases	Increases
Ionisation enthalpy	Decreases	Decreases	Decreases	Decreases (still very high)
Electronegativity	Decreases	Decreases	Decreases	Not usually assigned
Metallic character	Increases (N,P non-metal → As,Sb metalloid → Bi metal)	Increases (O,S non-metal → Se,Te metalloid → Po metal)	All non-metals; slight metallic in I / At	All non-metals
Melting / boiling point	MP: rises to As, falls to Bi; BP: increases	Increases (O₂ gas → Po solid)	Increases (F₂,Cl₂ gas → Br₂ liquid → I₂ solid)	Increases (all monatomic gases)
Electron gain enthalpy	Less negative overall (unlike Gr 16/17)	O less negative than S (small size, e⁻–e⁻ repulsion)	Cl more negative than F ⚠️ (F small size)	Positive (do not accept e⁻)

Key irregularities NEET loves

🔥Cl has more negative ΔH_eg than F — small size of F ⇒ electron–electron repulsion in the compact 2p subshell.
🔥O has less negative ΔH_eg than S — same reason (small O atom).
🔥Group 15 IE₁ order: N > P > As > Sb < Bi — Bi break due to poor shielding; extra-stable half-filled 2p³ raises N.
Group 15 melting point: N < P < As > Sb > Bi (peak at As).
Bond dissociation enthalpy of X₂: Cl₂ > Br₂ > F₂ > I₂ — F₂ low due to small size / lone-pair repulsion.

2 Oxidation States & Inert-Pair Effect

Group	Common OS	Trend / notes
15	−3, +3, +5	+5 stability decreases down (Bi⁺³ stable, Bi⁺⁵ strong oxidiser) — inert pair effect on 6s²
16	−2, +2, +4, +6	+6 stability decreases down; Po prefers +2/+4; O only ±2 (no d-orbitals)
17	−1, +1, +3, +5, +7	F: only −1 (no d-orbitals, most EN); others show +1 to +7 (odd values, use d-orbitals)
18	0 (mostly); Xe: +2, +4, +6, +8	Only Xe and Kr form compounds. XeO₄ has +8. Rn is radioactive.

🔥Inert pair effect: reluctance of ns² pair to bond ⇒ lower OS more stable down the group. Seen in Bi, Pb, Tl.
PbCl₄ decomposes to PbCl₂ + Cl₂ on heating; PbCl₂ is stable. Similarly BiCl₅ doesn't exist as isolable.
NCl₃ exists but NF₅ / NCl₅ do NOT (N has no d-orbitals). PCl₅ exists (P uses 3d).
F never shows positive OS. Cl → +7 in HClO₄; I → +7 in IF₇ / HIO₄.

3 Anomalous Behaviour of the First Element One-mark magnet

First element	Why it's anomalous	Specific NCERT anomalies
N (Group 15)	Small size · high EN · no d-orbitals · pπ–pπ multiple bonding	Only element to form pπ–pπ triple bond (N≡N gas, others solid); no pentahalide (NCl₅ etc. absent); NH₃ is basic, others less basic; N does not catenate significantly
O (Group 16)	Small size · high EN · no d-orbitals · strong pπ–pπ bonds	O₂ is a gas (S₈ solid); H₂O liquid (H-bonding) vs H₂S gas; only −2 and −1 (in peroxide) as OS; no expanded octet (no OF₆)
F (Group 17)	Smallest · most EN · no d-orbitals · low bond dissociation	Only OS = −1; HF is liquid (H-bonded); F₂ has anomalously low bond enthalpy; no oxoacids of the +7 type; forms strong H-bonds
He (Group 18)	1s² fully filled · smallest · very light	Does not form any known compound; used in cryogenics (bp 4.2 K); does not solidify at atm pressure

4 One-liner Facts & Exceptions MCQ-ready

🔥Group 15 hydrides thermal stability: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃ (BiH₃ least stable — bond energy falls down).
🔥Group 15 hydrides basic strength: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃ (matches lone-pair availability).
Group 15 hydrides bond angle: NH₃ 107.8° > PH₃ 93.6° > AsH₃ 91.8° > SbH₃ 91.3° > BiH₃ 90°.
Group 15 hydrides reducing character: BiH₃ > SbH₃ > AsH₃ > PH₃ > NH₃ (reverse of stability).
🔥Group 16 hydrides acidic order: H₂O < H₂S < H₂Se < H₂Te < H₂Po (bigger, weaker H–X bond ⇒ easier release of H⁺).
Group 16 hydrides thermal stability: H₂O > H₂S > H₂Se > H₂Te.
Group 16 hydrides reducing character: H₂O < H₂S < H₂Se < H₂Te.
Group 16 hydrides bond angle: H₂O 104.5° > H₂S 92° > H₂Se 91° > H₂Te 90°.
🔥Halogens oxidising power: F₂ > Cl₂ > Br₂ > I₂. Reducing character of X⁻: I⁻ > Br⁻ > Cl⁻ > F⁻.
HX acidic strength: HF < HCl < HBr < HI (bond enthalpy dominates, not EN).
HX bond enthalpy: HF > HCl > HBr > HI. HF liquid only one due to H-bonding.
🔥Halogen oxoacid acidity (same halogen): HXO < HXO₂ < HXO₃ < HXO₄ (higher OS ⇒ stronger acid).
Halogen oxoacid acidity (same OS +1): HOCl > HOBr > HOI.
ClF₃, BrF₃, IF₅, IF₇ are the main interhalogens tested. IF₇ has pentagonal bipyramidal geometry, sp³d³.
Noble gas compounds: only Xe and (limited) Kr. Xenon fluorides XeF₂, XeF₄, XeF₆.
Noble gas He boils at 4.2 K (lowest of all elements); Xe used in high-intensity photographic flashes.
N₂ triple bond dissociation enthalpy ≈ 941 kJ mol⁻¹ (highest for diatomic non-metal).
White P (P₄, tetrahedral, angle 60°) is more reactive than red P (polymeric, less reactive).
🔥Ammonia industrial: Haber process, 200 atm, 700 K, Fe catalyst + Mo promoter.
🔥HNO₃ industrial: Ostwald process — NH₃ → NO → NO₂ → HNO₃. Pt/Rh catalyst at 500 K.
H₂SO₄ industrial: Contact process. Key step: 2SO₂ + O₂ ⇌ 2SO₃ over V₂O₅ at 720 K.
Bleaching action of Cl₂: oxidation (permanent); of SO₂: reduction (temporary).
Ozone O₃ bond angle: 117°. Structure: bent, with resonance. sp² central O.
Ozone estimated by iodometry: 2 KI + O₃ + H₂O → 2 KOH + I₂ + O₂.
P₄O₁₀ + H₂O → H₃PO₄ (drying agent for gases except NH₃).
All group 18 elements are monatomic and colourless; Kr, Xe, Rn discovered after Ar. Ar is most abundant noble gas in air (~0.93 %).

5 Preparation & Key Reactions (balanced)

Group 15 — Ammonia (NH₃)

Haber: N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g) · 200 atm · 700 K · Fe/Mo

Lab: 2 NH₄Cl + Ca(OH)₂ → CaCl₂ + 2 NH₃ + 2 H₂O

Basic: NH₃ + HCl → NH₄Cl (white fumes — test)

With CuSO₄: 4 NH₃ + Cu²⁺ → [Cu(NH₃)₄]²⁺ (deep blue)

Group 15 — Nitric Acid (HNO₃)

Ostwald step 1: 4 NH₃ + 5 O₂ →(Pt/Rh, 500 K) 4 NO + 6 H₂O

Ostwald step 2: 2 NO + O₂ → 2 NO₂

Ostwald step 3: 3 NO₂ + H₂O → 2 HNO₃ + NO

Brown ring test: [Fe(H₂O)₅NO]²⁺ (brown ring at H₂SO₄ interface for NO₃⁻)

Group 15 — Phosphorus compounds

PCl₃ hydrolysis: PCl₃ + 3 H₂O → H₃PO₃ + 3 HCl

PCl₅ hydrolysis: PCl₅ + 4 H₂O → H₃PO₄ + 5 HCl

PCl₅ thermal: PCl₅ ⇌ PCl₃ + Cl₂ (dissociates on heating)

Group 16 — Sulphur compounds

Contact: S + O₂ → SO₂ ; 2 SO₂ + O₂ ⇌(V₂O₅, 720 K) 2 SO₃

Oleum: SO₃ + H₂SO₄ → H₂S₂O₇ ; H₂S₂O₇ + H₂O → 2 H₂SO₄

SO₂ + Cl₂ + activated C: SO₂ + Cl₂ → SO₂Cl₂ (sulphuryl chloride)

Group 16 — Ozone (O₃)

Silent electric discharge: 3 O₂ ⇌ 2 O₃ (endothermic; ΔH = +142 kJ/mol)

Estimation: 2 KI + O₃ + H₂O → 2 KOH + I₂ + O₂

Group 17 — Chlorine (Cl₂)

Deacon: 4 HCl + O₂ →(CuCl₂, 723 K) 2 Cl₂ + 2 H₂O

Lab (MnO₂): MnO₂ + 4 HCl → MnCl₂ + Cl₂ + 2 H₂O

With NaOH (cold, dilute): Cl₂ + 2 NaOH → NaCl + NaOCl + H₂O

With NaOH (hot, conc): 3 Cl₂ + 6 NaOH → 5 NaCl + NaClO₃ + 3 H₂O

Bleaching powder: 2 Ca(OH)₂ + 2 Cl₂ → Ca(OCl)₂ + CaCl₂ + 2 H₂O

Group 17 — HCl (lab)

Lab prep: NaCl + H₂SO₄(conc, 420 K) → NaHSO₄ + HCl

Group 18 — Xenon fluorides

XeF₂: Xe + F₂ →(400 °C, 1 atm, 1:5) XeF₂

XeF₄: Xe + 2 F₂ →(400 °C, 6 atm, 1:5) XeF₄

XeF₆: Xe + 3 F₂ →(300 °C, 60–70 atm, 1:20) XeF₆

Hydrolysis: XeF₆ + 3 H₂O → XeO₃ + 6 HF (also 2 XeF₆ + SiO₂ → 2 XeOF₄ + SiF₄)

6 Structures, Hybridisation & Shape

Compound	Central atom	Hyb.	Shape	Bond angle	LP on central
NH₃	N	sp³	Trigonal pyramidal	107.8°	1
PCl₅ (gas)	P	sp³d	Trigonal bipyramidal	90° / 120°	0
PCl₅ (solid)	—	—	[PCl₄]⁺ (sp³, td) + [PCl₆]⁻ (sp³d², oct)	109.5° / 90°	0 / 0
P₄	P	sp³	Tetrahedral cluster	60° (P–P–P)	1 each
P₄O₆	P	sp³	Cage: 4 P at corners, 6 O bridges	—	1 each P
P₄O₁₀	P	sp³	Like P₄O₆ + 4 terminal P=O	—	0 (all bonded)
SO₂	S	sp²	Bent	119.5°	1
SO₃	S	sp²	Trigonal planar	120°	0
H₂SO₄	S	sp³	Tetrahedral	109.5°	0
SF₆	S	sp³d²	Octahedral	90°	0
SF₄	S	sp³d	See-saw (distorted TBP)	—	1 (equatorial)
O₃	O	sp²	Bent (V-shape)	117°	1
ClF₃	Cl	sp³d	T-shape	87.5°	2
BrF₅	Br	sp³d²	Square pyramidal	90°	1
IF₇	I	sp³d³	Pentagonal bipyramidal	72° / 90°	0
XeF₂	Xe	sp³d	Linear	180°	3
XeF₄	Xe	sp³d²	Square planar	90°	2
XeF₆	Xe	sp³d³	Distorted octahedral	—	1
XeOF₄	Xe	sp³d²	Square pyramidal	—	1
XeO₃	Xe	sp³	Trigonal pyramidal	—	1
XeO₄	Xe	sp³	Tetrahedral	109.5°	0

7 Oxoacids

Group 15 oxoacids of P

Name	Formula	OS of P	Basicity	P–H bonds	Notes
Hypophosphorous	H₃PO₂ (H₂PO₂ + 1 P–H)	+1	Monobasic	2	Strong reducing
Phosphorous	H₃PO₃	+3	Dibasic	1	Reducing (has P–H)
Metaphosphoric	(HPO₃)ₙ	+5	Polymeric	0	—
Pyrophosphoric	H₄P₂O₇	+5	Tetrabasic	0	—
Orthophosphoric	H₃PO₄	+5	Tribasic	0	Most stable P oxoacid

Rule: Basicity = number of O–H (not P–H). Reducing property needs at least one P–H bond.

Group 16 oxoacids of S

Name	Formula	OS of S	Basicity
Sulphurous	H₂SO₃	+4	Dibasic
Sulphuric	H₂SO₄	+6	Dibasic (strong)
Pyrosulphuric (oleum)	H₂S₂O₇	+6	Dibasic
Thiosulphuric	H₂S₂O₃	+2 (avg)	Dibasic
Peroxymonosulphuric (Caro's)	H₂SO₅	+6	Dibasic (peroxo)
Peroxydisulphuric (Marshall's)	H₂S₂O₈	+6	Dibasic (peroxo)

Group 17 oxoacids of Cl

Name	Formula	OS of Cl	Basicity
Hypochlorous	HOCl	+1	Monobasic
Chlorous	HClO₂	+3	Monobasic
Chloric	HClO₃	+5	Monobasic
Perchloric	HClO₄	+7	Monobasic (strongest halogen oxoacid)

Acidic strength: HClO₄ > HClO₃ > HClO₂ > HOCl (more O ⇒ better delocalisation of –ve charge on conjugate base).

8 Hydrides · Oxides · Halides — Comparison

Group 15 hydrides EH₃

Property	Order
Thermal stability	NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃
Basicity	NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃
Reducing power	BiH₃ > SbH₃ > AsH₃ > PH₃ > NH₃
Boiling point	NH₃ > SbH₃ > AsH₃ > PH₃ (irreg. — H-bonding in NH₃)
Bond angle (E–H–E)	107.8° · 93.6° · 91.8° · 91.3° · 90°

Group 16 hydrides H₂E

Property	Order
Thermal stability	H₂O > H₂S > H₂Se > H₂Te > H₂Po
Acidic strength	H₂O < H₂S < H₂Se < H₂Te (opposite of stability)
Reducing power	H₂O < H₂S < H₂Se < H₂Te
Bond angle	104.5° · 92° · 91° · 90°
Boiling point	H₂O > H₂Te > H₂Se > H₂S (H₂O anomaly = H-bond)

Group 17 halides / hydrides HX

Property	Order
Bond dissociation enthalpy (HX)	HF > HCl > HBr > HI
Acidic strength (aq.)	HF < HCl < HBr < HI
Boiling point	HF > HI > HBr > HCl (HF H-bonded)
Reducing power	HF < HCl < HBr < HI
Oxidising power of X₂	F₂ > Cl₂ > Br₂ > I₂
Bond enthalpy (X₂)	Cl₂ > Br₂ > F₂ > I₂ (F₂ anomaly)

9 Colours · Smells · Physical Cues One-mark magnets

NO — colourless; NO₂ — brown; N₂O₃ — blue liquid; N₂O₄ — colourless.
Cl₂ — greenish-yellow; Br₂ — reddish-brown liquid; I₂ — violet solid, violet vapour.
F₂ — pale yellow gas; ClF₃ — colourless gas; ICl — red-brown solid; ICl₃ — orange solid.
SO₂ — colourless, pungent, suffocating smell; H₂S — rotten-egg smell; SO₃ — colourless.
O₃ — pale blue gas, characteristic pungent (chlorine-like) smell.
NH₃ — colourless, characteristic sharp smell; HCl gas — colourless, pungent, white fumes with NH₃.
P₄ (white) — waxy, translucent, garlic-like smell, glows in dark (chemiluminescence).
Red P — dark red / violet, non-poisonous, no glow.
Xenon fluorides all colourless solids. XeO₃ colourless explosive solid.
Rn — colourless, radioactive; He, Ne, Ar, Kr, Xe all colourless.
Bleaching powder — pale yellow, chlorine-like smell.
Aqua regia (3 HCl : 1 HNO₃) — orange fumes, dissolves Au and Pt.

10 Numbers & Values

Item	Value
N≡N bond enthalpy	≈ 941 kJ mol⁻¹
Ammonia synthesis (Haber)	200 atm · 700 K · Fe catalyst · Mo promoter
Contact process temp	720 K over V₂O₅
Ostwald catalyst temp	500 K over Pt/Rh gauze
Ozone formation ΔH	+142 kJ mol⁻¹ (endothermic)
NH₃ bond angle	107.8°
H₂O bond angle	104.5°
O₃ bond angle	117°
SO₂ bond angle	119.5°
P₄ bond angle (P–P–P)	60°
He boiling point	4.2 K (lowest of all elements)
Ar abundance in dry air	≈ 0.93 %
XeF₆ synthesis	Xe : F₂ = 1 : 20, 300 °C, 60–70 atm
HClO₄ Kₐ	Very large (strongest halogen oxoacid)
Halogens diatomic bond enthalpy order	Cl₂ 242 > Br₂ 192 > F₂ 158 > I₂ 151 (kJ/mol)

11 Trap Pairs / Easily Confused

H₃PO₃ dibasic vs H₃PO₄ tribasic — basicity counts O–H, not H (P–H doesn't ionise).

PCl₃ vs PCl₅ hydrolysis — H₃PO₃ vs H₃PO₄. Water molecules needed: 3 vs 4.

SO₂ bleaching (temporary, reduction) vs Cl₂ bleaching (permanent, oxidation).

F₂ oxidises most but HF is weakest acid — bond enthalpy dominates HX acidity.

ΔH_eg: Cl > F, S > O (not the other way!) — small-atom repulsion.

PCl₅ gas (TBP) vs PCl₅ solid ([PCl₄]⁺[PCl₆]⁻) — different structures.

XeF₂ sp³d linear (3 LP) vs XeF₄ sp³d² square-planar (2 LP).

NCl₃ (exists) vs NF₅ / NCl₅ (do NOT exist) — no d-orbital in N.

H₂O highest bp in Gr 16 hydrides, but lowest acidity — H-bonding effect vs bond strength.

Bond enthalpy of X₂: Cl > Br > F > I. F₂ is the anomaly (not smallest ⇒ strongest).

Brown ring test (nitrate) vs Ring test with H₂SO₄ interface. Complex is [Fe(H₂O)₅NO]²⁺.

White P (P₄) reactive, poisonous vs Red P polymeric, non-poisonous.

N₂O laughing gas vs NO colourless vs NO₂ brown.

H₂SO₄ dehydrating agent (removes H₂O from sugar → C).

HClO₄ strongest, HOCl weakest oxoacid of Cl — more O ⇒ more acidic.

12 Mnemonics

Group 15 elements — "Nothing Plus Ask Sb Billy" → N, P, As, Sb, Bi.

Group 16 (chalcogens) — "Our School Selects Tens Points" → O, S, Se, Te, Po.

Group 17 (halogens) — "Fast Clever Brown Indian Athlete" → F, Cl, Br, I, At.

Group 18 (noble) — "Help Needed At Krishna's Xerography Rn" → He, Ne, Ar, Kr, Xe, Rn.

Bond angle of Group 15 hydrides — "NH₃ = 107.8, drops to nearly 90 by BiH₃". Just N is odd; rest ≈ 90°.

Reducing power of Gr 15 hydrides = reverse of stability. Say: "BiH₃ boasts, NH₃ humbles."

Halogen oxoacid acidic order — "HO-Cl less to more O = more acidic". HOCl < HClO₂ < HClO₃ < HClO₄.

Colour of Cl₂, Br₂, I₂ — "Green Yellow, Red Brown, Violet" — like traffic lights going darker.

Xe fluoride LP count — XeF₂ = 3, XeF₄ = 2, XeF₆ = 1. "【2,54 cm】3, 2, 1 count-down."

Contact process: "V₂O₅ vanquishes oxygen at 720 K". Haber: "Fe/Mo" at "200 atm / 700 K".

13 Cross-Topic Connections

Chemical Bonding → hybridisation (sp³, sp³d, sp³d², sp³d³), VSEPR, back-bonding (BF₃, SO₂).
Redox → oxidising/reducing behaviour of halogens, ozone, HNO₃, H₂SO₄. Balancing in acidic / basic media.
Equilibrium → contact process, Haber process — Le Chatelier arguments for T and P.
s-Block → contrast basicity of NH₃ with alkali hydroxides; peroxide/superoxide of Na, K vs H₂O₂.
d-Block → V₂O₅ (contact), Pt/Rh (Ostwald), Fe (Haber) — catalyst identity is scored.
Coordination compounds → [Cu(NH₃)₄]²⁺ blue, [Fe(H₂O)₅NO]²⁺ brown ring (both use NH₃ / NO from Gr 15).
Environmental chem → SO₂/NO₂ acid rain, ozone hole (CFC → ClO• radicals).
Thermodynamics → ozone formation endothermic (+142 kJ/mol); F₂ anomalously low bond enthalpy.

14 High-Yield Summary — Top 15 Master these first

NH₃ bond angle 107.8°; drops sharply to ≈ 90° for PH₃ / AsH₃ / SbH₃ / BiH₃.
Stability: NH₃ > … > BiH₃. Reducing power: BiH₃ > … > NH₃. Opposite orders.
H₂O bond angle 104.5°; H₂S 92°; H₂Se 91°; H₂Te 90°. H₂O highest bp (H-bond).
Acidic strength of H₂E: H₂O < H₂S < H₂Se < H₂Te. Opposite of stability.
Halogen oxoacids: HClO₄ > HClO₃ > HClO₂ > HOCl. More O ⇒ stronger acid.
HX acidic order: HF < HCl < HBr < HI. Bond enthalpy dominates.
ΔH_eg: Cl > F, S > O (small-atom repulsion anomaly).
Bond enthalpy X₂: Cl₂ > Br₂ > F₂ > I₂. F₂ anomaly.
Haber: 200 atm, 700 K, Fe/Mo. Contact: V₂O₅, 720 K. Ostwald: Pt/Rh, 500 K.
PCl₅ solid = [PCl₄]⁺[PCl₆]⁻ (sp³ + sp³d²). Gas = TBP (sp³d).
Xe compounds: XeF₂ linear, XeF₄ square-planar, XeF₆ distorted-oct, XeO₃ pyramidal, XeO₄ tetrahedral.
Interhalogen shapes: ClF₃ T-shape, BrF₅ square-pyramidal, IF₇ pentagonal-bipyramidal.
O₃ bond angle 117°, sp²; ozone estimated by 2 KI + O₃ + H₂O → 2 KOH + I₂ + O₂.
Bleaching: Cl₂ oxidation (permanent), SO₂ reduction (temporary).
Basicity of P oxoacids = number of O–H (H₃PO₂ = 1, H₃PO₃ = 2, H₃PO₄ = 3). Reducing needs P–H.

15 Self-Test — 15 MCQs

Q1. Bond angle in NH₃ is:

(a) 90° (b) 104.5° (c) 107.8° (d) 120°

Answer

Q2. The catalyst used in the Contact process is:

(a) Fe + Mo (b) Pt/Rh (c) V₂O₅ (d) Al₂O₃

Answer

Q3. Which is the strongest acid?

(a) HOCl (b) HClO₂ (c) HClO₃ (d) HClO₄

Answer

(d) HClO₄ — highest OS of Cl (+7), best delocalisation of negative charge on ClO₄⁻.

Q4. Shape of XeF₄ is:

(a) Tetrahedral (b) Square planar (c) See-saw (d) Trigonal bipyramidal

Answer

(b) Square planar — sp³d², 2 lone pairs occupy axial positions.

Q5. Reducing power order for Gr 15 hydrides:

(a) NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃ (b) BiH₃ > SbH₃ > AsH₃ > PH₃ > NH₃ (c) NH₃ > BiH₃ > PH₃ (d) PH₃ > NH₃ > AsH₃

Answer

(b) BiH₃ > SbH₃ > AsH₃ > PH₃ > NH₃ — decreasing bond enthalpy makes E–H bond easier to break.

Q6. Basicity of H₃PO₃ is:

(a) 1 (b) 2 (c) 3 (d) 4

Answer

(b) 2 — H₃PO₃ has 2 O–H bonds and 1 P–H bond; only O–H ionise.

Q7. Which halogen has the lowest bond dissociation enthalpy (X₂)?

(a) F₂ (b) Cl₂ (c) Br₂ (d) I₂

Answer

(d) I₂ — 151 kJ/mol. Note F₂ (158 kJ/mol) is anomalously low too but I₂ is the lowest overall.

Q8. The brown ring test for nitrate involves the formation of:

(a) [Fe(H₂O)₆]²⁺ (b) [Fe(H₂O)₅NO]²⁺ (c) [Fe(NO)₆]³⁻ (d) FeNO₃

Answer

(b) [Fe(H₂O)₅NO]²⁺ — brown-ring complex at the H₂SO₄ / test-solution interface.

Q9. Which of the following does NOT exist?

(a) NCl₃ (b) PCl₅ (c) SF₆ (d) NF₅

Answer

(d) NF₅ — N has no d-orbital in the valence shell; cannot expand octet.

Q10. The reddish-brown gas evolved when Cu reacts with conc. HNO₃ is:

(a) NO (b) N₂O (c) NO₂ (d) N₂O₄

Answer

Q11. Which oxide of nitrogen is neutral?

(a) N₂O (b) NO₂ (c) N₂O₃ (d) N₂O₅

Answer

(a) N₂O — neutral (also NO is neutral). N₂O₃, N₂O₅ are acidic anhydrides.

Q12. IF₇ has hybridisation:

(a) sp³d (b) sp³d² (c) sp³d³ (d) sp³

Answer

Q13. The correct order of acidic strength of Gr 16 hydrides is:

(a) H₂O > H₂S > H₂Se > H₂Te (b) H₂Te > H₂Se > H₂S > H₂O (c) H₂S > H₂O > H₂Se > H₂Te (d) H₂Se > H₂S > H₂Te > H₂O

Answer

(b) H₂Te > H₂Se > H₂S > H₂O — E–H bond weakens down the group.

Q14. Which noble gas is used to obtain the lowest temperatures?

(a) He (b) Ne (c) Ar (d) Xe

Answer

(a) He — bp 4.2 K, used in cryogenics.

Q15. Assertion: PCl₅ exists but NCl₅ does not. Reason: N has no d-orbitals in its valence shell.

(a) Both A and R true, R is correct explanation (b) Both true, R is NOT correct explanation (c) A true, R false (d) A false, R true

Answer

(a) Both true, and the absence of d-orbital in N's valence shell (n = 2) is precisely why it cannot expand its octet to form NCl₅.

Card built 2026-07-20 under the "top-score Inorganic" plan. All content aligned strictly to NCERT Class 12 Ch 7 (p-Block Elements). No beyond-NCERT items flagged in this build.

📇 Group-wise Slides — p-Block Groups 15 · 16 · 17 · 18

📅 Built: 2026-07-20 One slide per group

Focused single-slide revision for each of the four p-Block groups on the NEET syllabus. Each slide is self-contained — periodic trends, anomaly of first element, one-liner facts, structures & hybridisation, colours & smells, mnemonics, traps, and a 5-question mini self-test. Use these when revising one group at a time.

🔵 Group 15 — Nitrogen family 🟢 Group 16 — Oxygen family 🟡 Group 17 — Halogens 🟣 Group 18 — Noble gases

Group 15 — Nitrogen Family (N · P · As · Sb · Bi)

Pnictogens · ns² np³ · valence 3, 5

📅 2026-07-20

Trends Down the Group

Property	Trend	Note
Atomic radius	Increases	Big jump N→P (2s→3s)
IE₁	Decreases	Order: N > P > As > Sb < Bi (irregular at Bi)
Electronegativity	Decreases	N most electronegative (3.0)
Metallic character	Increases	N,P non-metal · As,Sb metalloid · Bi metal
MP	Rises to As, then falls to Bi	Peak at As

Oxidation States & Inert-Pair

Common OS: −3, +3, +5. Down the group, +5 stability decreases; +3 becomes more stable (inert-pair effect on 6s²).
Bi³⁺ stable, Bi⁵⁺ strong oxidiser. Similarly PbCl₄ decomposes to PbCl₂ + Cl₂.
N shows +1, +2, +3, +4, +5 in various oxides/oxoacids (NO₂ = +4, HNO₃ = +5, N₂O = +1, etc.).

Anomalous Behaviour of N

Small size · high EN · no d-orbitals · strong pπ–pπ multiple bonds.
Only N forms N≡N triple bond — P, As, Sb, Bi exist as tetrahedral P₄ or polymeric solids.
No NCl₅ / NF₅ (no d-orbitals); PCl₅, PF₅ exist normally.
NH₃ is a strong base (H-bonded, liquefies easily); PH₃/AsH₃ etc. are progressively weaker.
N does not catenate significantly; P/S catenate more readily.

Hydrides & Trends

Property	Order
Thermal stability	NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃
Basicity	NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃
Reducing power	BiH₃ > SbH₃ > AsH₃ > PH₃ > NH₃ (reverse of stability)
Bond angle	NH₃ 107.8° · PH₃ 93.6° · AsH₃ 91.8° · SbH₃ 91.3° · BiH₃ 90°
Boiling point	NH₃ (H-bond) > SbH₃ > AsH₃ > PH₃

Key Preparations (balanced)

NH₃ (Haber): N₂ + 3 H₂ ⇌ 2 NH₃ · 200 atm · 700 K · Fe/Mo

HNO₃ (Ostwald): 4 NH₃ + 5 O₂ → 4 NO + 6 H₂O (Pt/Rh, 500 K); 2 NO + O₂ → 2 NO₂; 3 NO₂ + H₂O → 2 HNO₃ + NO

PCl₃ hydrolysis: PCl₃ + 3 H₂O → H₃PO₃ + 3 HCl

PCl₅ hydrolysis: PCl₅ + 4 H₂O → H₃PO₄ + 5 HCl

Brown-ring test: [Fe(H₂O)₅NO]²⁺ (nitrate detection at H₂SO₄ interface)

Structures & Hybridisation

Species	Hyb	Shape	Bond angle
NH₃	sp³	Trigonal pyramidal	107.8°
PCl₅ (g)	sp³d	Trigonal bipyramidal	90° / 120°
PCl₅ (s)	[PCl₄]⁺ sp³ + [PCl₆]⁻ sp³d²	Td + Oct	109.5° / 90°
P₄	sp³	Tetrahedral cluster	60° P–P–P
P₄O₁₀	sp³	Cage + 4 terminal P=O	—

P Oxoacids (basicity from O–H count)

Acid	Formula	OS of P	Basicity	Reducing?
Hypophosphorous	H₃PO₂	+1	Monobasic	Strong (2 P–H)
Phosphorous	H₃PO₃	+3	Dibasic	Yes (1 P–H)
Orthophosphoric	H₃PO₄	+5	Tribasic	No

Colours & Physical Cues

NO — colourless · NO₂ — brown · N₂O₃ — blue liquid · N₂O — laughing gas (colourless).
White P (P₄) — waxy, garlic smell, glows in dark (chemiluminescence), poisonous.
Red P — dark red, polymeric, non-poisonous, no glow.
NH₃ — pungent smell, white fumes with HCl (test).

Mnemonic — Order of Group 15: "Nothing Plus Ask Sb Billy" → N, P, As, Sb, Bi.

Bond angle drop: NH₃ = 107.8°, all others ≈ 90° (say "N is odd; rest is 90").

Trap: H₃PO₃ is dibasic (not tribasic!) — the P–H bond doesn't ionise. Similarly H₃PO₂ is monobasic.

Trap: PCl₅ gas = TBP (sp³d); PCl₅ solid = [PCl₄]⁺[PCl₆]⁻ (sp³ + sp³d²) — different structures.

Mini Self-Test (5 Q)

Q1. Bond angle in PH₃ ≈

(a) 90° (b) 93.6° (c) 107.8° (d) 120°

Answer

(b) 93.6° — much smaller than NH₃'s 107.8° due to poor sp³ hybridisation in larger P.

Q2. Which does NOT exist?

(a) NCl₃ (b) NF₃ (c) NCl₅ (d) NH₃

Answer

Q3. Reducing power of Gr 15 hydrides is largest for:

(a) NH₃ (b) PH₃ (c) AsH₃ (d) BiH₃

Answer

(d) BiH₃ — bond enthalpy decreases down the group, so BiH₃ releases H atoms most easily.

Q4. Basicity of H₃PO₂ is

(a) 1 (b) 2 (c) 3 (d) 4

Answer

(a) 1 — only 1 O–H bond; the 2 P–H bonds don't ionise.

Q5. Haber process operating conditions:

(a) 200 atm, 700 K, Fe/Mo (b) 1 atm, 300 K, Pt (c) 100 atm, 500 K, V₂O₅ (d) 500 atm, 900 K, Ni

Answer

(a) 200 atm, 700 K, Fe catalyst + Mo promoter.

Group 16 — Oxygen Family / Chalcogens (O · S · Se · Te · Po)

ns² np⁴ · valence 2, 4, 6

📅 2026-07-20

Trends Down the Group

Property	Trend	Note
Atomic radius	Increases	Largest jump O → S
IE₁	Decreases	O less than N — half-filled 2p³ of N is more stable
Electronegativity	Decreases	O = 3.5 (2nd only to F)
ΔH_eg (electron gain)	Anomalous	\|ΔH_eg\| of S > O (O small ⇒ e⁻–e⁻ repulsion)
Metallic character	Increases	O,S non-metal · Se,Te metalloid · Po metal
MP / BP	Increases	O₂ gas → Po solid

Oxidation States

Common: −2, +2, +4, +6. Down the group +6 stability decreases; Po prefers +2/+4.
Oxygen only shows −2 (and −1 in peroxide, +2 in OF₂, −½ in superoxide). No expanded octet (no OF₆).

Anomalous Behaviour of O

Small size · high EN · no d-orbitals · strong pπ–pπ bonds.
O₂ is a gas (S₈ solid, ring structure). H₂O is liquid due to H-bonding; H₂S is a gas.
Only −2 as common OS (no +4/+6). No expanded-octet compounds.

Hydrides & Trends

Property	Order
Thermal stability	H₂O > H₂S > H₂Se > H₂Te > H₂Po
Acidic strength	H₂O < H₂S < H₂Se < H₂Te (opposite of stability)
Reducing power	H₂O < H₂S < H₂Se < H₂Te
Bond angle	H₂O 104.5° · H₂S 92° · H₂Se 91° · H₂Te 90°
Boiling point	H₂O (H-bond, anomaly) > H₂Te > H₂Se > H₂S

Key Preparations (balanced)

H₂SO₄ (Contact): S + O₂ → SO₂; 2 SO₂ + O₂ ⇌ 2 SO₃ (V₂O₅, 720 K); SO₃ + H₂SO₄ → H₂S₂O₇ (oleum); + H₂O → 2 H₂SO₄

Ozone (silent electric discharge): 3 O₂ ⇌ 2 O₃ · ΔH = +142 kJ/mol (endothermic)

Ozone estimation: 2 KI + O₃ + H₂O → 2 KOH + I₂ + O₂

Structures & Hybridisation

Species	Hyb	Shape	Bond angle
O₃	sp²	Bent (V-shape), resonance	117°
SO₂	sp²	Bent	119.5°
SO₃	sp²	Trigonal planar	120°
H₂SO₄	sp³	Tetrahedral	109.5°
SF₆	sp³d²	Octahedral (very stable, inert)	90°
SF₄	sp³d	See-saw (1 LP equatorial)	—

S Oxoacids

Name	Formula	OS of S	Notes
Sulphurous	H₂SO₃	+4	Dibasic; unstable (mostly aq. only)
Sulphuric	H₂SO₄	+6	Dibasic, strong; dehydrating & oxidising
Pyrosulphuric (oleum)	H₂S₂O₇	+6	From SO₃ + H₂SO₄
Peroxydisulphuric (Marshall's)	H₂S₂O₈	+6	Contains –O–O– bond

Colours & Physical Cues

SO₂ — colourless, pungent, suffocating smell; H₂S — rotten-egg smell.
O₃ — pale blue gas, sharp chlorine-like smell.
Rhombic S₈ — yellow, stable; monoclinic S₈ — pale yellow, above 96 °C.
SO₂ bleaches by reduction (temporary); Cl₂ bleaches by oxidation (permanent).

Mnemonic — Order of Group 16: "Our School Selects Tens Points" → O, S, Se, Te, Po.

Contact vs Haber: Contact = SO₂ oxidation, V₂O₅, 720 K. Haber = N₂+H₂, Fe/Mo, 200 atm, 700 K.

Trap: ΔH_eg magnitude — Cl > F, S > O (not the other way!) — small atom means e⁻–e⁻ repulsion.

Trap: H₂O has highest boiling point in Gr 16 but LOWEST acidity (H-bond dominates in bp, bond enthalpy dominates in acidity).

Mini Self-Test (5 Q)

Q1. Ozone bond angle:

(a) 104.5° (b) 109.5° (c) 117° (d) 120°

Answer

Q2. Order of acidic strength of Gr 16 hydrides:

(a) H₂O > H₂S > H₂Se > H₂Te (b) H₂Te > H₂Se > H₂S > H₂O (c) H₂S > H₂O > H₂Se (d) H₂O = H₂S

Answer

(b) H₂Te > H₂Se > H₂S > H₂O — E–H bond weakens down the group.

Q3. Contact process catalyst is:

(a) Fe/Mo (b) V₂O₅ (c) Pt/Rh (d) Ni

Answer

(b) V₂O₅ at 720 K for 2 SO₂ + O₂ ⇌ 2 SO₃.

Q4. SF₄ shape is:

(a) Tetrahedral (b) See-saw (c) Octahedral (d) Square planar

Answer

(b) See-saw — sp³d with 1 lone pair in equatorial position.

Q5. Bleaching by SO₂ is:

(a) Oxidation, permanent (b) Reduction, temporary (c) Oxidation, temporary (d) Reduction, permanent

Answer

(b) Reduction, temporary — colour returns on standing (re-oxidation by air).

Group 17 — Halogens (F · Cl · Br · I · At)

ns² np⁵ · valence 1 (with variable OS via d)

📅 2026-07-20

Trends Down the Group

Property	Trend	Note
Atomic radius	Increases	Smallest of the period (across); increases down
IE₁	Decreases	Halogens have highest IE₁ in period (except noble)
Electronegativity	Decreases	F = 4.0 (highest of all elements)
ΔH_eg (electron gain)	Anomalous	\|ΔH_eg\| of Cl > F — F small, e⁻–e⁻ repulsion in 2p
Physical state	Trend	F₂ gas · Cl₂ gas · Br₂ liquid · I₂ solid
Bond enthalpy of X₂	Anomalous	Cl₂ > Br₂ > F₂ > I₂ (F₂ low due to lone-pair repulsion)

Oxidation States

F: only −1 (no d-orbitals, most EN — never positive).
Cl, Br, I: −1, +1, +3, +5, +7 (use d-orbitals for higher OS).
+7 is the highest common OS (HClO₄, IF₇, HIO₄).

Anomalous Behaviour of F

Smallest halogen · most EN of all elements · no d-orbitals in valence shell.
Only shows −1; never positive OS.
F₂ bond enthalpy anomalously low (158 kJ/mol) due to lone-pair repulsion in tiny F₂.
HF liquid due to H-bonding; other HX gases.
No oxoacids of +7 type; only limited oxoacids known for F.
Strongest oxidiser of all elements.

HX & Halogen Trends

Property	Order
Oxidising power (X₂)	F₂ > Cl₂ > Br₂ > I₂
Reducing power (X⁻)	I⁻ > Br⁻ > Cl⁻ > F⁻
HX bond dissociation enthalpy	HF > HCl > HBr > HI
HX acidic strength (aq.)	HF < HCl < HBr < HI (bond enthalpy dominates, not EN!)
HX boiling point	HF > HI > HBr > HCl (HF H-bonded, anomaly)
Oxoacid acidity (same X)	HXO < HXO₂ < HXO₃ < HXO₄
Oxoacid acidity (same +1 OS)	HOCl > HOBr > HOI

Key Preparations (balanced)

Cl₂ (Deacon): 4 HCl + O₂ → 2 Cl₂ + 2 H₂O (CuCl₂, 723 K)

Cl₂ (lab, MnO₂): MnO₂ + 4 HCl → MnCl₂ + Cl₂ + 2 H₂O

Cl₂ + cold, dil. NaOH: Cl₂ + 2 NaOH → NaCl + NaOCl + H₂O

Cl₂ + hot, conc. NaOH: 3 Cl₂ + 6 NaOH → 5 NaCl + NaClO₃ + 3 H₂O

Bleaching powder: 2 Ca(OH)₂ + 2 Cl₂ → Ca(OCl)₂ + CaCl₂ + 2 H₂O

Interhalogens & Structures

Species	Hyb	Shape	Bond angle
ClF₃	sp³d	T-shape	87.5°
BrF₃	sp³d	T-shape	—
IF₅	sp³d²	Square pyramidal (1 LP)	—
BrF₅	sp³d²	Square pyramidal	90°
IF₇	sp³d³	Pentagonal bipyramidal (0 LP)	72° / 90°

Colours & Physical Cues

F₂ — pale yellow gas · Cl₂ — greenish-yellow gas · Br₂ — reddish-brown liquid · I₂ — violet solid, violet vapour.
Bleaching powder — pale yellow, chlorine-like smell.
Aqua regia (3 HCl : 1 HNO₃) — orange fumes, dissolves Au and Pt.

Mnemonic — Halogens: "Fast Clever Brown Indian Athlete" → F, Cl, Br, I, At.

Oxoacid acidity: "More O = more acid" (delocalisation of charge on conjugate base).

Trap: HF is the weakest HX acid despite F being most EN — because bond enthalpy dominates, not EN.

Trap: F₂ has lowest bond enthalpy (not highest!) — lone-pair repulsion in the tiny F₂ molecule.

Mini Self-Test (5 Q)

Q1. Correct order of acidic strength of HX:

(a) HF > HCl > HBr > HI (b) HI > HBr > HCl > HF (c) HCl > HF > HBr > HI (d) All equal

Answer

(b) HI is strongest; HF is weakest — bond enthalpy decreases down the group.

Q2. Shape of IF₇ is:

(a) Tetrahedral (b) Octahedral (c) Pentagonal bipyramidal (d) Trigonal bipyramidal

Answer

Q3. The strongest halogen oxoacid is:

(a) HOCl (b) HClO₂ (c) HClO₃ (d) HClO₄

Answer

(d) HClO₄ — Cl is in +7; charge on ClO₄⁻ delocalised over 4 O atoms.

Q4. Which halogen shows only −1 oxidation state?

(a) Cl (b) Br (c) I (d) F

Answer

(d) F — most electronegative and has no d-orbitals in the valence shell.

Q5. Product of Cl₂ + hot conc. NaOH:

(a) NaCl + NaOCl (b) NaCl + NaClO₃ (c) NaClO₄ only (d) Cl₂ dissolves without reaction

Answer

(b) 3 Cl₂ + 6 NaOH → 5 NaCl + NaClO₃ + 3 H₂O (disproportionation).

Group 18 — Noble Gases (He · Ne · Ar · Kr · Xe · Rn)

ns² np⁶ (He: 1s²) · closed shell · monatomic

📅 2026-07-20

Trends Down the Group

Property	Trend	Note
Atomic radius	Increases	Largest in each period (van der Waals)
IE₁	Decreases	Still highest in period; He has the highest IE₁ of all elements
Boiling point	Increases	He 4.2 K (lowest of all elements) · Rn ~ 211 K
Chemical reactivity	Increases	Only Xe (and limited Kr) form compounds; He, Ne, Ar do not
Abundance in air	Ar > Ne > He > Kr > Xe	Ar ≈ 0.93 % of dry air

Why Noble Gases Are Inert

Completely filled valence shell (ns²np⁶; He is 1s²) — no tendency to gain, lose or share electrons.
Very high IE₁ — highest in each period.
Positive electron gain enthalpy — do not accept electrons.
Reactivity emerges only for Xe (and marginally Kr) because larger size ⇒ lower IE ⇒ can be forced to bond with the most electronegative element (F, O).

Xenon Compounds — Preparations (balanced)

XeF₂: Xe + F₂ → XeF₂ (400 °C, 1 atm, Xe:F₂ = 1:5)

XeF₄: Xe + 2 F₂ → XeF₄ (400 °C, 6 atm, Xe:F₂ = 1:5)

XeF₆: Xe + 3 F₂ → XeF₆ (300 °C, 60–70 atm, Xe:F₂ = 1:20)

Hydrolysis of XeF₆: XeF₆ + 3 H₂O → XeO₃ + 6 HF

Partial hydrolysis: XeF₆ + H₂O → XeOF₄ + 2 HF; 2 XeF₆ + SiO₂ → 2 XeOF₄ + SiF₄

XeO₄ (rare): Ba₃XeO₆ + H₂SO₄ → XeO₄ + …

Structures & Hybridisation

Species	Hyb	Shape	LP on Xe	Bond angle
XeF₂	sp³d	Linear	3	180°
XeF₄	sp³d²	Square planar	2	90°
XeF₆	sp³d³	Distorted octahedral	1	—
XeOF₄	sp³d²	Square pyramidal	1	—
XeO₃	sp³	Trigonal pyramidal	1	—
XeO₄	sp³	Tetrahedral	0	109.5°

Uses (NCERT-listed)

He — cryogenics, filling weather balloons (safer than H₂), diving mixtures (heliox), MRI-magnet coolant.
Ne — advertising signs (red-orange glow), high-voltage indicators, aircraft lighting.
Ar — inert atmosphere in welding & metallurgy; fill filament bulbs to prevent oxidation.
Kr — high-intensity lamps for airport runways.
Xe — high-intensity photographic flash lamps, arc lamps.
Rn — radioactive; formerly used in cancer radiotherapy.

Colours & Physical Cues

All noble gases are colourless, odourless, tasteless, monatomic gases.
Xenon fluorides: colourless solids (XeF₂, XeF₄, XeF₆).
XeO₃ — colourless explosive solid; XeO₄ — colourless gas, explosive.
Neon signs glow red-orange when a discharge is passed through Ne.

Mnemonic — Noble gases: "Help Needed At Krishna's Xerography Rn" → He, Ne, Ar, Kr, Xe, Rn.

Xe fluoride LP count: "3, 2, 1 count-down" — XeF₂ has 3 LP, XeF₄ has 2 LP, XeF₆ has 1 LP.

Trap: He does NOT solidify at atmospheric pressure — needs applied pressure. Boiling point 4.2 K is the lowest of any element.

Trap: XeF₄ is square planar (not tetrahedral!) — 2 lone pairs occupy axial positions in sp³d² geometry.

Mini Self-Test (5 Q)

Q1. Shape of XeF₂ is:

(a) Linear (b) Bent (c) Square planar (d) T-shape

Answer

(a) Linear — sp³d with 3 lone pairs occupying equatorial positions of TBP.

Q2. Which noble gas has the lowest boiling point?

(a) He (b) Ne (c) Ar (d) Xe

Answer

(a) He — 4.2 K (lowest boiling point of any element).

Q3. Product of XeF₆ + H₂O (complete hydrolysis):

(a) XeO₃ + HF (b) XeO₄ + HF (c) Xe + O₂ (d) XeOF₄ + HF

Answer

(a) XeF₆ + 3 H₂O → XeO₃ + 6 HF (complete hydrolysis). Partial hydrolysis gives XeOF₄.

Q4. Number of lone pairs on central Xe in XeF₆:

(a) 0 (b) 1 (c) 2 (d) 3

Answer

(b) 1 — sp³d³ hybridisation, 6 bond pairs + 1 lone pair ⇒ distorted octahedral.

Q5. Which noble gas is used in fluorescent-lamp red-orange signs?

(a) He (b) Ne (c) Ar (d) Kr

Answer

(b) Ne — the classic "neon-sign" red-orange glow comes from Ne discharge.

All four slides built 2026-07-20 under the "top-score Inorganic" plan. Content strictly from NCERT Class 12 Ch 7. Use these when revising one group at a time; the combined pack (above) is the cross-group overview.

⚖️ Quick Comparisons — Side-by-Side Reference

The simple idea

NEET / JEE loves to test the difference between two almost-identical concepts in p-Block Elements. These 10 side-by-side tables show every common pair. Read each row across; the one you forget is the one they'll ask.

VS Group 15: Nitrogen vs Phosphorus (anomalous N)

Property	Nitrogen (N)	Phosphorus (P)
Allotropy	Only N₂	White, red, black
Bond type	pπ-pπ multiple bonds	Mostly single bonds; uses d-orbitals for π
Catenation	No	Yes (P₄ tetrahedron, polyphosphates)
Hydride basicity	NH₃ strongly basic	PH₃ weakly basic
Halides	NCl₃, NF₃ exist	PCl₃, PCl₅ (uses d-orbital)
Max oxidation state	+5 (HNO₃, NO₃⁻)	+5 (uses d-orbital)

VS Group 16: Oxygen vs Sulphur

Property	Oxygen	Sulphur
State at STP	Gas (O₂, O₃)	Solid (S₈ ring)
Bond type	pπ-pπ double bond (O=O)	Single bonds, S₈ ring
Electronegativity	3.5	2.5
Acid strength of hydride	H₂O weak	H₂S stronger
Catenation	Limited	Extensive (polysulphides)
Max OS	+2 (OF₂)	+6 (SF₆, H₂SO₄)

VS Group 17: Halogens trends (F → I)

Property	F	Cl	Br	I
Phase at STP	Gas (pale yellow)	Gas (yellow-green)	Liquid (red-brown)	Solid (purple-black)
Bond dissociation (X-X)	Lower (F-F anomaly)	Highest	Medium	Lowest
Oxidising power	Strongest	High	Lower	Lowest
Electronegativity	Highest (4.0)	3.0	2.8	2.5
Hydride (HX) acidity	Weakest (HF)	Stronger	Stronger	Strongest (HI)

F-F bond is anomalously weak due to lone-pair repulsion in small atoms.

VS Inert pair effect: stability of oxidation states

Group	Higher OS stable in	Lower OS stable in
13 (Tl)	Lighter (B, Al, Ga): +3	Heavier (Tl): +1
14 (Pb)	Lighter (C, Si): +4	Heavier (Pb): +2
15 (Bi)	Lighter (N, P): +5	Heavier (Bi): +3

Heavier p-block atoms prefer the OS that is 2 less than group oxidation state — 'inert pair' of 6s² electrons is reluctant to be ionised.

VS Oxoacids of chlorine — strength & OS

Acid	Cl OS	Acid strength	Oxidising power
HClO (hypochlorous)	+1	Weakest	Strongest
HClO₂ (chlorous)	+3	Weak	Strong
HClO₃ (chloric)	+5	Strong	Moderate
HClO₄ (perchloric)	+7	Strongest	Weakest

More O atoms → stronger acid (better stabilisation of conjugate base). But oxidising power DROPS as you go up the OS for the same element.

VS Diagonal relationships

Pair	Common property	Why
Li ↔ Mg	Both form nitrides, hard carbonates	Similar charge density
Be ↔ Al	Amphoteric oxides; form covalent halides	Similar polarising power
B ↔ Si	Similar oxides (acidic), form similar halides	Similar size and EN

VS Oxidising power: F₂ vs O₃ vs KMnO₄

Oxidiser	E° (V)	Typical use
F₂	+2.87	Strongest known oxidiser
O₃	+2.07	Bleaching, oxidising organic
MnO₄⁻ (acidic)	+1.51	Permanganate titrations
Cr₂O₇²⁻ (acidic)	+1.33	Dichromate titrations
O₂	+1.23	Common

VS Allotropes of phosphorus and sulphur

Element	Major allotropes	Differences
P (phosphorus)	White P₄ (tetrahedral, reactive); Red P (polymeric, stable)	White ignites in air; Red doesn't
S (sulphur)	Rhombic (yellow, stable below 95.6°C); Monoclinic (above 95.6°C)	Both contain S₈ rings
S	Plastic sulphur	Long polymeric chains; metastable

VS Noble gases: Group 18 properties down the group

Property	He	Ne	Ar	Kr	Xe	Rn
Atomic number	2	10	18	36	54	86
Boiling point (K)	4.2	27	87	120	165	211
Compounds known?	No	No	Few	Few	Many (XeF₂, XeOF₄)	Few

Xenon's chemistry is the richest because its ionisation energy is low enough for F to oxidise it.

VS NH₃ vs PH₃ vs AsH₃

Property	NH₃	PH₃	AsH₃
Boiling point (°C)	−33	−87	−55 (rises due to mass)
H-bonding	Yes	No	No
Basic strength	Strong	Weak	Very weak
Bond angle	107°	93.5°	91.8°
Reducing power	Weakest	Strong	Stronger

📈Graphs Every NEET Aspirant Must Recognise

1 · Bond dissociation energy of X₂ (halogens)

Trend order: Cl₂ > Br₂ > F₂ > I₂. F₂ is the anomaly — despite being smallest, its BDE is lower than Cl₂ or Br₂ due to lone-pair repulsion in the compact F-F distance. This is why F₂ is so reactive.

2 · Acid strength of HX (hydrohalic acids)

HF < HCl < HBr < HI. Down the group, H-X bond weakens → easier ionisation → stronger acid. HF is anomalously weak (H-bonding stabilises undissociated HF).

3 · Boiling point of hydrides (H-bonding anomaly)

The hydride BP zigzag: NH₃, H₂O, HF all show anomalously high BPs vs their group. Reason: strong H-bonding due to small size + high EN of N, O, F. Same graph applies to Groups 15, 16, 17.

4 · Acidity of oxoacids of chlorine

Acidity increases as oxidation state increases. More O atoms delocalise the − charge on the conjugate base → stabler → stronger acid. HClO₄ is one of the strongest acids known.

5 · Xenon fluoride shapes — VSEPR summary

XeF₂ linear (sp³d), XeF₄ square planar (sp³d²), XeF₆ distorted octahedral (sp³d³). Structure follows VSEPR: count bp + lp, apply "electron-pair geometry vs molecular geometry" distinction. Every NEET-year has one Q on this table.

6 · Ionisation enthalpy of Group 15 (half-filled anomaly)

IE₁ decreases smoothly down the group (size increases). Within a period, however, N's IE is higher than expected due to extra stability of half-filled 2p³ configuration.

⚡Shortcuts & Memory Tricks (15)

Shortcut 1 — First-element anomalies

N, O, F all break their group's pattern. Reason (same for all): small size + high EN + no d-orbitals. Learn this rule once, apply 3 times.

Shortcut 2 — H₃PO₃ dibasic trap

Basicity of any oxoacid = number of O-H bonds only. H₃PO₃ has 3 H's, but only 2 attached to O; 1 is P-H (doesn't ionise) → dibasic. Guaranteed NEET question.

Shortcut 3 — HX acid order

HF < HCl < HBr < HI. Bond strength weakens down → easier ionisation. HF weakest despite F being most EN — H-bonding traps it.

Shortcut 4 — HX bond enthalpy order

HF > HCl > HBr > HI (decreasing). Consistent with acid strength (weaker bond = stronger acid).

Shortcut 5 — X₂ bond dissociation zigzag

Cl₂ > Br₂ > F₂ > I₂. F₂ is the anomaly — small size → lp-lp repulsion. This is why F₂ is so reactive despite low BDE.

Shortcut 6 — Oxoacid acidity increases with oxidation state

HOCl < HClO₂ < HClO₃ < HClO₄. More O = more resonance stabilisation of anion = stronger acid. HClO₄ is a super acid.

Shortcut 7 — SO₂ bleaching is reversible

SO₂ bleaches by reduction (donates nascent H). Colour returns on standing. Cl₂ bleaches by oxidation — permanent. Classic contrast question.

Shortcut 8 — PCl₅ solid vs gas

Gas: trigonal bipyramidal (sp³d). Solid: ionic [PCl₄]⁺[PCl₆]⁻ — tetrahedral cation + octahedral anion. NEET's #1 phosphorus halide trap.

Shortcut 9 — Ozone bond angle

O₃ is bent, 117°. Both O-O bonds equal (resonance). Bond order = 1.5. Same 117° angle for SO₂ and ClO₂ (bent triatomics).

Shortcut 10 — Ammonia weaker base than expected? No — stronger.

Basicity order: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃. Small N has concentrated lone pair, most available for donation. Trend reverses for bond angle (see #11).

Shortcut 11 — Bond angle order for XH₃ (Group 15)

NH₃ (107°) > PH₃ (94°) > AsH₃ (92°) > SbH₃ (91°) > BiH₃ (~90°). Down the group, orbital s-character drops → bond angle approaches 90°.

Shortcut 12 — Interhalogen geometry

Follow VSEPR: XX' = linear, XX'₃ = T-shape (2 lp), XX'₅ = square pyramidal (1 lp), XX'₇ = pentagonal bipyramidal. Larger halogen is always central.

Shortcut 13 — Xenon fluorides in one line

XeF₂ linear (3 lp) · XeF₄ sq planar (2 lp) · XeF₆ distorted octahedral (1 lp). All 3 hybridisations differ: sp³d, sp³d², sp³d³.

Shortcut 14 — NO₃⁻ vs NO₂⁻ shape

NO₃⁻ = trigonal planar (sp²), N has no lone pair. NO₂⁻ = bent (sp²), N has one lone pair. Confusing because both are "sp²".

Shortcut 15 — He and Ne have NO chemistry

Only Xe (and to lesser extent Kr and Rn) form compounds. He, Ne, Ar are so-far chemically inert. Reason: Xe has lower IE (electron farther from nucleus).

⚠️Exceptions & NEET Traps

The 12 exceptions NEET returns to again and again

H₃PO₃ is dibasic, not tribasic — only 2 O-H bonds. The 1 P-H doesn't ionise.
H₃PO₂ is monobasic, has 2 P-H bonds (both non-ionising). Only 1 O-H.
PCl₅ is trigonal bipyramidal in gas but ionic [PCl₄]⁺[PCl₆]⁻ in solid.
NCl₃ exists but NCl₅ doesn't: N has no d-orbitals → max covalency = 4.
F₂ has LOWER bond dissociation energy than Cl₂ and Br₂ due to lone-pair repulsion in the compact F-F bond.
HF is the weakest hydrohalic acid despite F being most electronegative — strong H-bonding stabilises undissociated HF.
Electron affinity: Cl > F. Adding an electron to small F suffers extra repulsion.
SO₂ bleaches temporarily (by reduction); Cl₂ bleaches permanently (by oxidation). Opposite mechanisms.
Ozone (O₃) has both O-O bonds equal (1.28 Å) — resonance hybrid, not distinct single + double.
Basicity trend NH₃ > PH₃ but bond angle NH₃ > PH₃ — reversed reasoning: basicity depends on lp availability; bond angle depends on s-character.
Only Xe (with Kr, Rn) forms compounds. He, Ne, Ar have no confirmed stable compounds.
N₂ molecule is exceptionally inert despite N being reactive as an atom — reason: N≡N bond has 945 kJ/mol dissociation energy, one of the strongest known.

🔥Most-Asked NEET Topics (Frequency-Ranked)

Based on frequency of appearance in NEET/AIPMT questions over the last 10-15 years. Top 5 give ~ 70% of the chapter's questions.

Rank	Topic	Frequency	Specific angles NEET uses
1	Xenon fluoride shapes + hybridisation	🔥🔥🔥🔥🔥 · every year	XeF₂ linear · XeF₄ sq planar · XeF₆ distorted oct · XeOF₄ sq pyramidal
2	Oxoacids of P/Cl — basicity + acidity	🔥🔥🔥🔥 · 1-2/yr	H₃PO₃ dibasic trap · HClO₄ > HClO₃ > HClO₂ > HOCl
3	Interhalogens — structure + reactivity	🔥🔥🔥🔥 · 1-2/yr	ClF₃ T-shape · IF₇ pentagonal bipyramidal · BrF₅ sq pyramid
4	HX acid + bond strength order	🔥🔥🔥 · 1/yr	HF weakest acid · HI strongest · Bond enthalpy HF > HCl > HBr > HI
5	PCl₃, PCl₅ + halides	🔥🔥🔥 · 1/yr	PCl₅ ionic in solid state · PCl₃ pyramidal hydrolysis
6	Ozone structure + reactivity	🔥🔥 · every 2 yrs	Bent 117° · KI test · bond order 1.5
7	H₂SO₄ (contact process + reactions)	🔥🔥🔥 · 1/yr	Oxidising vs dehydrating · V₂O₅ catalyst
8	Sulphur allotropes + SO₂	🔥🔥 · every 2 yrs	Rhombic vs monoclinic · SO₂ bleaching by reduction
9	NH₃, HNO₃, brown ring test	🔥🔥🔥 · 1/yr	Haber process · Ostwald · [Fe(H₂O)₅NO]²⁺ brown ring
10	Anomalous behaviour of N, O, F	🔥🔥 · every 2 yrs	No d-orbitals · pπ-pπ multiple bonds · max covalency

🎯Assertion-Reason Practice Bank

10 NEET-style A-R drills. Both statements may be true — you must decide whether R correctly explains A.

A1. H₃PO₃ is dibasic. R1. It has 2 O-H bonds and 1 P-H bond.

Answer + explanation

Both A and R are true, and R correctly explains A. Basicity = ionisable H count = O-H count only. H₃PO₃ has 2 O-H, so dibasic. ✓

A2. HF is the weakest hydrohalic acid. R2. F is the most electronegative element.

Answer + explanation

Both A and R are true, but R does NOT correctly explain A. HF weakness is due to strong H-F bond enthalpy (568 kJ/mol) and H-bonding stabilising undissociated HF — NOT directly due to F's electronegativity.

A3. F₂ has lower bond dissociation energy than Cl₂. R3. Lone-pair-lone-pair repulsion is significant in the small F-F molecule.

Answer + explanation

Both A and R are true, and R correctly explains A. F₂ BDE = 159 kJ/mol vs Cl₂ = 243. Small F-F distance forces the 6 lone pairs on the two F atoms close together → strong repulsion → weakens the bond. ✓

A4. XeF₄ is square planar. R4. Xe in XeF₄ is sp³d² hybridised with 2 lone pairs.

Answer + explanation

Both A and R are true, and R correctly explains A. sp³d² gives 6 electron pairs; 4 bp F + 2 lp on axial positions gives sq planar geometry (VSEPR). ✓

A5. Ozone bleaches indigo. R5. Ozone is a reducing agent.

Answer + explanation

A is true; R is FALSE. Ozone is a powerful oxidising agent (releases nascent O). It bleaches by oxidation.

A6. NH₃ has a lower BP than PH₃ predicts. R6. N is small and electronegative, so NH₃ shows strong H-bonding.

Answer + explanation

A is FALSE; R is true. NH₃ actually has an anomalously HIGH BP (−33 °C) vs PH₃ (−88 °C) precisely because of H-bonding. Read A carefully — "lower than predicts" is wrong.

A7. HClO₄ is a stronger acid than HOCl. R7. Increased number of oxygen atoms delocalises negative charge on the conjugate base.

Answer + explanation

Both A and R are true, and R correctly explains A. ClO₄⁻ has 4 O atoms delocalising the −1 charge; OCl⁻ has 1. Stabler anion = stronger acid. ✓

A8. PCl₅ exists in solid state as ionic [PCl₄]⁺[PCl₆]⁻. R8. The [PCl₆]⁻ anion is octahedral (sp³d²).

Answer + explanation

Both A and R are true, but R doesn't cause A — R is a factual statement about the anion's shape. The ionic structure exists because of packing efficiency in the solid. Mark: both true, R not the explanation.

A9. Xe forms compounds while He, Ne, Ar do not. R9. Xe has lower ionisation enthalpy than He/Ne/Ar.

Answer + explanation

Both A and R are true, and R correctly explains A. Xe IE₁ = 1170 kJ/mol vs He 2372, Ne 2080, Ar 1520. Lower IE = easier to share/donate e⁻ to strongly EN F → forms XeF₂, XeF₄, etc. ✓

A10. SO₂ can bleach a coloured flower, and the colour returns on standing. R10. SO₂ bleaches by reduction (via nascent H), which is a reversible process.

Answer + explanation

Both A and R are true, and R correctly explains A. Reduction bleaching is reversible — atmospheric oxidation restores the original chromophore. ✓ Contrast: Cl₂ bleaches by irreversible oxidation.

🎯Final Checklist Before Chapter Test

☐ H₃PO₃ dibasic vs H₃PO₄ tribasic memorised?
☐ PCl₅ solid state = [PCl₄]⁺[PCl₆]⁻ ionic — recalled cold?
☐ HX acid order (HF weakest → HI strongest) explained by bond weakness?
☐ F₂ BDE anomaly (Cl₂ > Br₂ > F₂ > I₂) with lp-lp reason?
☐ HClO₄ > HClO₃ > HClO₂ > HOCl acidity ladder + reason?
☐ SO₂ bleaches by reduction (reversible) · Cl₂ by oxidation (permanent) contrast?
☐ Ozone bent 117°, bond order 1.5, powerful oxidiser?
☐ Interhalogen shapes: XX' linear, XX'₃ T, XX'₅ sq pyramid, XX'₇ pentagonal bipyramid?
☐ XeF₂ linear · XeF₄ sq planar · XeF₆ distorted oct — hybridisations cold?
☐ Anomaly of N/O/F explained by (small size + no d-orbitals + high EN) — one rule for all three?
☐ Brown-ring test for NO₃⁻ (FeSO₄ + conc H₂SO₄ → brown [Fe(H₂O)₅NO]²⁺)?
☐ Haber vs Ostwald vs Contact process — which for what?

All 12 ticked → NEET-pace for this chapter. Any 3+ unchecked → repair those before Day 5 test.