Class 12 — p-Block Elements

Chapter Overview & Weightage

The p-block elements (Group 15 to 18) chapter is one of the highest-weightage in Class 12 Chemistry. Each year, this chapter contributes 8–10 marks to the board paper, often through reaction-based MCQs, structure-and-bonding questions, and a 5-mark long-answer combining preparation, properties, and uses.

CBSE Class 12 Weightage (Year-by-Year)

Year	Marks	Topics asked
2024	9	Group 15 oxoacids, interhalogens, noble gas compounds
2023	8	$H_2SO_4$ manufacture, $XeF_4$ structure
2022	10	$NH_3$ vs $PH_3$ , halogen reactivity, $XeF_2$
2021	8	Allotropes of phosphorus, ozone preparation

Key Concepts You Must Know

Group 15 (Nitrogen family): Trends in oxidation states, anomalous behaviour of nitrogen, allotropes of phosphorus, oxoacids ( $HNO_3$ , $H_3PO_4$ ).

Group 16 (Oxygen family): Allotropes of sulphur and oxygen (ozone), preparation and properties of sulphuric acid (contact process), oxoacids of sulphur.

Group 17 (Halogens): Trends in bond strength, reactivity, oxidising power. Interhalogen compounds, oxoacids of chlorine.

Group 18 (Noble gases): Compounds of Xe ( $XeF_2, XeF_4, XeF_6, XeO_3$ ). Structures using VSEPR.

Anomalous behaviour: Why N, O, F differ from heavier members of their groups. Small size, high electronegativity, no d-orbitals.

Important Reactions to Remember

Three steps:

$S + O_2 \to SO_2$
$2SO_2 + O_2 \xrightarrow{V_2O_5, 450°C} 2SO_3$
$SO_3 + H_2SO_4 \to H_2S_2O_7 \xrightarrow{H_2O} 2H_2SO_4$

The middle step is reversible — Le Chatelier’s principle is applied (low temperature + excess $O_2$ ).

$4NH_3 + 5O_2 \xrightarrow{Pt, 500°C} 4NO + 6H_2O$

$2NO + O_2 \to 2NO_2$

$3NO_2 + H_2O \to 2HNO_3 + NO$

Compound	Hybridisation	Lone pairs	Geometry
$XeF_2$	$sp^3d$	3	Linear
$XeF_4$	$sp^3d^2$	2	Square planar
$XeF_6$	$sp^3d^3$	1	Distorted octahedral
$XeO_3$	$sp^3$	1	Pyramidal
$XeOF_4$	$sp^3d^2$	1	Square pyramidal

Solved Previous Year Questions

PYQ 1 — CBSE 2024, 3 Marks

Why is $NH_3$ a weaker acid than $PH_3$ but a stronger base?

$NH_3$ a stronger base because nitrogen’s lone pair is more available (small size, high charge density). $PH_3$ a weaker base because the lone pair on the larger phosphorus atom is more diffuse and less available for donation. Acidity follows the opposite trend due to bond strength: P–H bond is weaker, so $PH_3$ ionises (slightly) more easily as an acid.

PYQ 2 — CBSE 2023, 5 Marks

Draw the structures of $XeF_4$ and $XeOF_4$ using VSEPR theory.

$XeF_4$ : $sp^3d^2$ , two lone pairs in axial positions, four F’s in a square plane. Square planar.

$XeOF_4$ : $sp^3d^2$ , one lone pair, four F’s around base, O at top. Square pyramidal with Xe at the centre.

PYQ 3 — CBSE 2022, 3 Marks

Why does fluorine show abnormal electron affinity compared to chlorine?

Fluorine’s electron affinity is less negative than chlorine’s despite F being more electronegative. Reason: F is small, so the incoming electron faces strong electron-electron repulsion in the compact 2p orbitals. Cl is larger, so the incoming electron settles in 3p with less repulsion.

Difficulty Distribution

Difficulty	% of Marks	Sub-topics
Easy	30%	Definitions, allotropes, simple reactions
Medium	50%	Trends, structures, oxoacids
Hard	20%	Anomalous behaviour, contact/Ostwald processes, noble gas chemistry

Expert Strategy

Week 1 — Group 15 thoroughly. Memorise $NH_3$ , $HNO_3$ preparation, oxoacids of phosphorus, and allotropes. Roughly 3 marks every year.

Week 2 — Group 16 + 17. Sulphuric acid manufacture (5-mark guarantee). Halogen reactivity trends. Interhalogen structures.

Week 3 — Group 18. Just Xe compounds — NCERT lists 5 of them with structures. Memorise them all in one sitting.

NCERT line method: Read each NCERT line on p-block as a potential exam answer. Boards stick very close to NCERT phrasing. If you can recite the textbook explanation for “why is $H_2O$ liquid but $H_2S$ gas?”, you’ll get full marks.

Common Traps

Trap 1: Confusing the trend in oxidising power vs reducing power.

Halogens: oxidising power $F > Cl > Br > I$ . But reducing power of $HX$ goes the opposite way: $HI > HBr > HCl > HF$ .

Trap 2: Wrong hybridisation for Xe compounds.

$XeF_4$ has $sp^3d^2$ (not $sp^3d$ ). $XeF_2$ has $sp^3d$ . Count: total electron pairs = bond pairs + lone pairs.

Trap 3: $H_3PO_3$ is dibasic, not tribasic.

Phosphorous acid has two —OH groups and one P—H bond. Only the —OH protons ionise. Many students assume all 3 H’s are acidic.

Trap 4: Catalyst confusion in $H_2SO_4$ manufacture.

Contact process uses $V_2O_5$ as catalyst, not Pt. Students confuse it with Ostwald process for $HNO_3$ , which uses Pt.

Trap 5: White vs red phosphorus reactivity.

White P is more reactive (strained tetrahedral $P_4$ , reacts at room temperature). Red P is less reactive (polymeric chain). Reactivity opposite to what students sometimes guess.