Chapter Overview & Weightage
p-Block Elements is one of the most consistently high-weightage chapters in JEE Chemistry. It spans Groups 13–18 and covers an enormous range — from the anomalous behavior of boron to the noble gas compounds that still surprise students.
p-Block gets 8–10% weightage in JEE Main (roughly 2–3 questions per paper). In JEE Advanced, expect conceptual questions on oxoacids, allotropes, and interhalogen compounds. This chapter rewards memorization done with understanding.
| Year | JEE Main Questions | Key Topics Tested |
|---|---|---|
| 2024 | 3 | Oxoacids of sulphur, interhalogen compounds |
| 2023 | 2 | Allotropes of carbon, Group 15 hydrides |
| 2022 | 3 | Boron anomaly, noble gas compounds, oxoacids |
| 2021 | 2 | Group 16 oxygen family, XeF compounds |
| 2020 | 3 | Halogen compounds, phosphorus allotropes |
The pattern is clear: oxoacids and interhalogen compounds appear almost every year. Allotropes and anomalous behavior are the secondary focus.
Key Concepts You Must Know
Prioritized by exam frequency — tackle these in order.
Group 15 (Nitrogen Family) — High Priority
- Oxidation states of N, P, As, Sb, Bi and their trends
- Hydrides (NH₃, PH₃): basicity decreases down the group, why PH₃ is weaker base than NH₃
- Oxoacids of nitrogen: HNO₂ (nitrous acid) vs HNO₃ (nitric acid) — structures and oxidising nature
- Oxoacids of phosphorus: H₃PO₂, H₃PO₃, H₃PO₄ — number of P=O bonds determines basicity, P–H bonds determine reducing nature
Group 16 (Oxygen Family) — High Priority
- Allotropes of sulphur: rhombic, monoclinic, plastic sulphur — stable form at room temperature is rhombic
- Oxoacids of sulphur: H₂SO₃, H₂SO₄, H₂S₂O₃ (thiosulphate), H₂S₂O₇ (oleum), H₂S₂O₈ (persulphuric acid)
- Ozone structure and its reactions with KI and PbS
Group 17 (Halogens) — High Priority
- Interhalogen compounds: types (AX, AX₃, AX₅, AX₇), hybridisation, geometry
- Oxoacids of chlorine: HOCl, HOClO, HOClO₂, HOClO₃ — acid strength increases with more oxygen atoms
- Fluorine’s anomalous behavior: no d-orbitals, highest electronegativity, why HF is weak acid
Group 13 (Boron Family) — Medium Priority
- Anomalous behavior of boron vs aluminium: diagonal relationship with silicon
- Borax structure, boron hydrides (diborane structure — banana bonds)
- Aluminium’s amphoteric nature
Group 14 (Carbon Family) — Medium Priority
- Allotropes of carbon: diamond, graphite, fullerene (C₆₀) — hybridisation and properties
- Silicates classification: ortho, pyro, chain, sheet, 3D
- Anomalous behavior of carbon: catenation, pπ-pπ bonding
Group 18 (Noble Gases) — Lower Priority but Scoring
- Xenon fluorides: XeF₂, XeF₄, XeF₆ — hybridisation and geometry
- Xenon oxyfluorides and oxides: XeOF₂, XeOF₄, XeO₃
Important Formulas
Basicity = number of OH groups directly bonded to P
- H₃PO₂ (hypophosphorous acid): basicity = 1 (one OH, two P–H, one P=O)
- H₃PO₃ (phosphorous acid): basicity = 2 (two OH, one P–H, one P=O)
- H₃PO₄ (phosphoric acid): basicity = 3 (three OH, one P=O)
When to use: Any question asking about number of replaceable hydrogen atoms.
For same central atom: acid strength increases with number of oxygen atoms (or oxidation state of central atom).
HClO < HClO₂ < HClO₃ < HClO₄
When to use: Comparing acid strengths within the same group’s oxoacids.
| Formula | Hybridisation | Geometry |
|---|---|---|
| XX’ (ClF, BrCl) | sp³ | Linear |
| XX’₃ (ClF₃) | sp³d | T-shaped |
| XX’₅ (BrF₅) | sp³d² | Square pyramidal |
| XX’₇ (IF₇) | sp³d³ | Pentagonal bipyramidal |
When to use: Any question on shape or hybridisation of interhalogen compounds.
| Compound | Hybridisation | Geometry | Lone pairs on Xe |
|---|---|---|---|
| XeF₂ | sp³d | Linear | 3 |
| XeF₄ | sp³d² | Square planar | 2 |
| XeF₆ | sp³d³ | Distorted octahedral | 1 |
When to use: Noble gas compound questions in JEE Main — these appear almost every alternate year.
B₂H₆ has four terminal B–H bonds (2-centre, 2-electron) and two bridging B–H–B bonds (3-centre, 2-electron, called banana bonds or τ-bonds).
Hybridisation of B in diborane = sp³
When to use: Questions on boron hydrides, banana bonds, or electron-deficient compounds.
Solved Previous Year Questions
PYQ 1 — JEE Main 2024 Shift 1
Q: Which of the following oxoacids of phosphorus is a diprotic acid?
A: H₃PO₃ (phosphorous acid)
Solution:
First, count the OH groups on phosphorus — that gives basicity (not the total H atoms in the formula).
H₃PO₃ has the structure: one P=O, one P–H, and two P–OH groups.
Only the OH hydrogens are replaceable (ionisable). The P–H bond is too strong to ionise under normal conditions.
So H₃PO₃ releases 2 protons → it is diprotic.
Compare: H₃PO₂ (hypophosphorous acid) has only one OH group → monoprotic. H₃PO₄ has three OH groups → triprotic.
Many students count all the H atoms in the formula and say H₃PO₃ is triprotic. Wrong. The P–H bond does NOT ionise. Only P–OH bonds contribute to basicity.
PYQ 2 — JEE Main 2022
Q: The correct order of bond angles in BF₃, PF₃, and ClF₃ is:
A: BF₃ (120°) > PF₃ (97.8°) > ClF₃ (87.5°)
Solution:
BF₃ is sp² hybridised, no lone pairs → ideal trigonal planar → 120°.
PF₃ is sp³ hybridised with one lone pair → lone pair–bond pair repulsion compresses the angle below 109.5° → 97.8°.
ClF₃ is sp³d hybridised with two lone pairs (T-shaped) → two lone pairs exert maximum repulsion → angle compresses to 87.5°.
The more lone pairs, the more compression of bond angle. This is a direct application of VSEPR theory.
For T-shaped molecules (like ClF₃, IF₃), the lone pairs occupy equatorial positions in the trigonal bipyramidal arrangement. This is a common JEE Advanced concept question.
PYQ 3 — JEE Main 2023
Q: Identify the allotrope of sulphur that is paramagnetic.
A: Plastic sulphur (also called amorphous or fibrous sulphur in some texts), but more precisely, the S₂ molecule (in vapour form above 1000°C) is paramagnetic.
Solution:
At very high temperatures, S₈ rings break down into S₂ molecules. S₂ is analogous to O₂ — it has two unpaired electrons in antibonding molecular orbitals, making it paramagnetic.
The stable solid allotropes — rhombic (α-sulphur) and monoclinic (β-sulphur) — contain S₈ rings with all electrons paired → diamagnetic.
Plastic sulphur has long spiral chains of S atoms, also diamagnetic.
The answer is: S₂ (vapour phase sulphur) is paramagnetic. This distinction is frequently tested.
Difficulty Distribution
For JEE Main, based on PYQ analysis over 5 years:
| Difficulty | Approximate % | What It Tests |
|---|---|---|
| Easy | 35% | Direct recall — oxoacid formulas, hybridisation of XeF compounds |
| Medium | 45% | Application — identifying acid strength order, bond angle comparisons |
| Hard | 20% | Multi-step reasoning — diborane bonding, silicate classification, anomalous behaviour reasons |
The “Hard” 20% in p-Block usually comes from JEE Advanced, not Main. For JEE Main, focus on the Easy-Medium questions — that’s where your marks are.
Expert Strategy
Week 1 — Build the Framework
Start with Groups 15, 16, 17 in that order — they have the most PYQ coverage. Don’t try to memorise every compound blindly. Instead, understand the central atom’s oxidation state and use that to predict properties.
Week 2 — Oxoacids Mastery
Make a single A4 sheet with all oxoacids: formula, structure sketch, basicity, reducing/oxidising nature. Oxoacids of sulphur and phosphorus together account for ~30% of p-Block questions in JEE Main.
Week 3 — Groups 13, 14, 18
Noble gas compounds are easy marks — limited compounds, clear patterns. Boron’s anomalous behavior and diborane structure need one focused session each.
Topper strategy: Solve PYQs topic-by-topic, not year-by-year. When you do all oxoacid PYQs together, you see the pattern instantly. Three questions on H₃PO₃ basicity in five years — that question is coming again.
Silicates — spend less time here
Silicate classification (ortho, pyro, chain, sheet, 3D framework) appears rarely in JEE Main. Learn the Si:O ratios (SiO₄⁴⁻, Si₂O₇⁶⁻, etc.) in one hour and move on.
Common Traps
Trap 1 — Fluorine “should” form higher oxoacids
Students assume fluorine forms HFO₃ or HOF₂ like other halogens. It doesn’t. Fluorine has no d-orbitals and the highest electronegativity — it cannot expand its octet. The only oxoacid of fluorine is HOF (hypofluorous acid), and it is unstable. All higher oxoacids (HOClO, HOClO₂, HOClO₃) are chlorine-based.
Trap 2 — Confusing reducing nature with basicity in phosphorus acids
H₃PO₂ (basicity = 1) has TWO P–H bonds → strong reducing agent. H₃PO₃ (basicity = 2) has ONE P–H bond → moderate reducing agent. H₃PO₄ (basicity = 3) has NO P–H bonds → not a reducing agent.
The P–H bond is what reduces (not the OH group). Students mix up “more H atoms = more reducing” — wrong. Count only the P–H bonds.
Trap 3 — XeF₆ geometry
XeF₆ has sp³d³ hybridisation with one lone pair. Students mark it as octahedral. The correct answer is distorted octahedral (or pentagonal bipyramidal in some descriptions). The lone pair distorts the shape. JEE examiners specifically ask for this distinction.
Trap 4 — Rhombic vs Monoclinic sulphur stability
Below 96°C → rhombic sulphur is stable. Above 96°C → monoclinic sulphur is stable. 96°C is the transition temperature.
Students often reverse this. Room temperature (25°C) < 96°C, so rhombic is the stable form you encounter in labs.
Trap 5 — Interhalogen vs Noble gas: which is more reactive?
Interhalogen compounds are generally more reactive than the parent halogens because the A–X bond (between two different halogens) is weaker than a homodiatomic halogen bond. This is counterintuitive — students assume noble gas compounds are more reactive because they involve “inert” atoms breaking their shell. In reality, XeF₂ is quite stable compared to ClF₃.
One last point on exam day: p-Block questions in JEE Main are rarely calculation-heavy. They test whether you’ve understood the exceptions and the logic, not whether you can crunch numbers. A student who understands why fluorine has no d-orbitals will always outperform one who’s memorised a table of properties without the reasoning.