p-Block Elements is one of the most heavily tested Inorganic Chemistry sections in both JEE and NEET. The chapter spans six groups (13 to 18) and covers a vast range of reactions, structures, and properties. Yet despite this breadth, the exam questions come from a surprisingly predictable set of concepts — anomalous behaviour of first members, oxoacid structures, trends down the group, and industrial processes.
This guide covers all six groups systematically — the trends that matter, the key reactions, the important compounds, and the specific facts that examiners reach for year after year. The final section lists the 8 traps that cost students marks even when they have studied the chapter thoroughly.
p-Block Elements contributes 3–5 questions in JEE Mains and 5–8 questions in NEET every year — making it one of the highest-weightage single topics in NEET Inorganic Chemistry. Groups 15, 16, and 17 are tested most intensively. Group 18 (noble gases) usually contributes 1 question.
Group 13 — The Boron Family
Group 13 elements (B, Al, Ga, In, Tl) have the general electronic configuration ns²np¹. The key theme across this group is the contrast between boron (a metalloid with covalent chemistry) and the rest (metals with ionic chemistry).
Anomalous Behaviour of Boron
Boron differs from aluminium and the rest of the group in several important ways — this is a direct exam question in NEET almost every year:
- Small size and high charge density: Boron forms covalent compounds; Al and below form ionic compounds.
- Electron deficiency: BX₃ compounds are Lewis acids — they accept electron pairs from donors. This makes BCl₃ a stronger Lewis acid than AlCl₃ (despite Al being more metallic).
- No d-orbitals in valence shell: Boron's maximum covalency is 4 (by accepting a lone pair to form BF₄⁻). It cannot expand its octet unlike heavier members.
- Diagonal relationship with Silicon: Boron and Si share many properties (both form covalent hydrides, both oxides are acidic glass-formers, both are semiconductors).
Key Boron Compounds
- Borax (Na₂B₄O₇·10H₂O): Contains the tetranuclear unit [B₄O₅(OH)₄]²⁻. On heating gives boric acid; used in borax bead test (characteristic coloured beads with transition metal oxides).
- Boric acid (H₃BO₃): A weak monobasic acid — but NOT a proton donor. It acts as a Lewis acid, accepting OH⁻ from water: B(OH)₃ + H₂O → [B(OH)₄]⁻ + H⁺. Planar structure with H-bonding between molecules.
- Diborane (B₂H₆): Has two types of B–H bonds — four terminal (2c-2e) and two bridging (3c-2e banana bonds). Bridging H atoms are not normal H-bridge bonds. Diborane hydrolyses in water to boric acid + H₂.
Group 14 — The Carbon Family
Group 14 (C, Si, Ge, Sn, Pb) shows the widest range from non-metal (C) to metal (Pb). Carbon's unique ability to catenate (form long chains with itself) sets it apart from all other elements. Silicon's chemistry is dominated by silicates and silicones.
- Carbon allotropes: Diamond (sp³, hardest natural substance), graphite (sp², electrical conductor, used as lubricant and electrode), fullerenes (C₆₀ buckminsterfullerene — truncated icosahedron).
- CO vs CO₂: CO is a reducing agent and a Lewis base (donates electron pair through C). CO₂ is an acidic oxide. CO is toxic because it binds haemoglobin 200× more strongly than O₂.
- SiO₂ structure: Each Si is tetrahedrally bonded to 4 oxygen atoms in a giant covalent structure. Unlike CO₂ (linear molecular gas), SiO₂ is a solid with very high melting point because Si cannot form pπ–pπ double bonds with O.
- Inert pair effect: Down the group, the +2 oxidation state becomes more stable relative to +4. Pb²⁺ is more stable than Pb⁴⁺; Sn⁴⁺ is more stable than Sn²⁺. PbO₂ is a strong oxidising agent because Pb prefers +2.
Group 15 — The Nitrogen Family
Group 15 (N, P, As, Sb, Bi) elements have ns²np³ configuration. The half-filled p subshell gives extra stability, making these elements less reactive than their neighbours. This group is the most heavily tested in JEE from the p-block.
Nitrogen vs Phosphorus — Key Contrasts
- N₂ is very stable (triple bond, bond energy 941 kJ/mol); P₄ is reactive (strained P–P single bonds in tetrahedral structure).
- Nitrogen cannot form d-orbital compounds — maximum covalency is 4 (in NH₄⁺). Phosphorus can expand octet: PCl₅ exists, NCl₅ does not.
- N forms pπ–pπ bonds (N₂, NO₂, HNO₃ have π bonds); P does not form stable pπ–pπ bonds due to larger size — instead forms pπ–dπ bonds.
Oxoacids of Nitrogen — Must Know All
Oxoacids of Phosphorus — Basicity Rule
The basicity of a phosphorus oxoacid equals the number of P–OH groups (not total OH groups). P–H bonds are non-ionisable.
- H₃PO₄ (Phosphoric acid): 3 P–OH groups → tribasic.
- H₃PO₃ (Phosphorous acid): 2 P–OH + 1 P–H → dibasic. Strong reducing agent (has P–H bond).
- H₃PO₂ (Hypophosphorous acid): 1 P–OH + 2 P–H → monobasic. Strongest reducing agent among P oxoacids.
p-Block Reactions Confusing You?
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Book Free DemoGroup 16 — The Chalcogens
Group 16 (O, S, Se, Te, Po) has ns²np⁴ configuration. Oxygen and sulphur are by far the most tested elements from this group.
Anomalous Behaviour of Oxygen
- Oxygen shows only –2, –1, and 0 oxidation states (no positive oxidation states, unlike S which can go up to +6). This is because oxygen has no d-orbitals and high electronegativity.
- Oxygen forms strong H-bonds due to its small size and high electronegativity — hence water has abnormally high boiling point.
- O₂ is paramagnetic (two unpaired electrons in π* orbitals) — explained by MOT, not Lewis structure.
Sulphur and Its Compounds
- SO₂: Angular molecule (lone pair on S), acts as both reducing agent and bleaching agent (by reduction, unlike Cl₂ which bleaches by oxidation). SO₂ is the intermediate in the Contact Process for H₂SO₄ manufacture.
- H₂SO₄ (Concentrated): Strong oxidising agent, dehydrating agent, and acid. Hot concentrated H₂SO₄ oxidises C to CO₂, S to SO₂, and P to H₃PO₄. It reacts with HBr and HI but not HCl (Cl₂ is produced instead).
- Oxoacids of Sulphur: H₂SO₃ (sulphurous, S = +4), H₂SO₄ (sulphuric, S = +6), H₂S₂O₇ (oleum/pyrosulphuric), H₂S₂O₈ (peroxodisulphuric — contains O–O peroxide link).
Group 17 — The Halogens
Group 17 (F, Cl, Br, I, At) is the most electronegative group. Halogens are among the most reactive non-metals. Fluorine is unique — its properties dominate a significant portion of exam questions from this group.
Anomalous Behaviour of Fluorine
- Most electronegative element — shows only –1 oxidation state (never positive, unlike Cl, Br, I which can show +1, +3, +5, +7).
- No d-orbitals — cannot form oxoacids (HClO, HClO₂, HClO₃, HClO₄ exist; HFO does not).
- Smallest halogen — forms the strongest H-bond (HF has the highest boiling point among hydrogen halides).
- HF is a weak acid (unlike HCl, HBr, HI which are strong acids) — due to the very strong H–F bond.
- F₂ is the strongest oxidising agent among halogens; oxidising power decreases down the group: F₂ > Cl₂ > Br₂ > I₂.
Interhalogen Compounds
Formed between two different halogens: XY, XY₃, XY₅, XY₇. The larger halogen is always the central atom. Examples: ClF, ClF₃, BrF₅, IF₇. These are more reactive than the parent halogens because the X–Y bond is weaker than X–X or Y–Y bonds.
Oxoacids of Chlorine
Group 18 — Noble Gases
Noble gases (He, Ne, Ar, Kr, Xe, Rn) have completely filled valence shells — ns²np⁶ (except He: 1s²). They are chemically inert under normal conditions. However, xenon does form compounds with fluorine and oxygen.
- Xenon fluorides: XeF₂ (linear, 3 lone pairs on Xe), XeF₄ (square planar, 2 lone pairs), XeF₆ (distorted octahedral, 1 lone pair). Structures follow VSEPR — the lone pairs count.
- XeO₃: Pyramidal (3 lone pairs on Xe). XeOF₄: square pyramidal.
- Uses: He — balloons, deep-sea diving; Ne — advertisement signs (red glow); Ar — inert atmosphere in welding; Kr/Xe — lighting, lasers.
Structure shortcut for XeF₂, XeF₄, XeF₆: Count total electron pairs around Xe (bonding + lone pairs), arrange by VSEPR, then identify shape from bonding pairs only. XeF₂: 5 total pairs (3 lone + 2 bond) → trigonal bipyramidal arrangement → linear shape. XeF₄: 6 total pairs (2 lone + 4 bond) → octahedral arrangement → square planar shape.
The 8 Traps Examiners Set Every Year
Saying BCl₃ is a Weaker Lewis Acid Than AlCl₃
BCl₃ is actually a stronger Lewis acid than AlCl₃. In BCl₃, the empty p-orbital on B is available for acceptance (less back-donation from Cl than in AlCl₃). AlCl₃ partially compensates through d-orbital involvement. Many students assume the more metallic element is the stronger Lewis acid — it is not.
Treating Boric Acid as a Proton Donor
H₃BO₃ is a Lewis acid, not a Brønsted acid. It does not donate a proton — it accepts OH⁻ from water. This is why it is a weak monobasic acid. The reaction is B(OH)₃ + H₂O → [B(OH)₄]⁻ + H⁺, not H₃BO₃ → H⁺ + H₂BO₃⁻.
Saying NCl₅ Exists
NCl₅ does not exist. Nitrogen has no d-orbitals in its valence shell, so it cannot expand its octet beyond 4 bonds. PCl₅ exists because phosphorus has available 3d-orbitals. This question — "why does PCl₅ exist but NCl₅ does not?" — appears in NEET almost every year.
Counting All OH Groups for Basicity of Phosphorus Oxoacids
Only P–OH groups contribute to basicity — P–H bonds are non-ionisable and do not count. H₃PO₃ has 3 H atoms but only 2 P–OH bonds → dibasic, not tribasic. Getting this wrong costs marks in every paper that tests phosphorus oxoacids.
Saying SO₂ Bleaches by Oxidation
SO₂ bleaches by reduction (it reduces coloured compounds to colourless ones). Cl₂ bleaches by oxidation. The bleaching by SO₂ is temporary — on exposure to air, the coloured compound is re-oxidised and colour returns. The bleaching by Cl₂ is permanent.
Saying HF is a Strong Acid
HF is a weak acid despite fluorine being the most electronegative element. The H–F bond is so strong (bond enthalpy 568 kJ/mol) that it does not dissociate fully in water. HCl, HBr, and HI are all strong acids. This reversal — weakest acid despite strongest electronegativity — is a very common NEET trap.
Confusing Thermal Stability with Acid Strength for Chlorine Oxoacids
Both thermal stability AND acid strength increase in the same order: HClO < HClO₂ < HClO₃ < HClO₄. Oxidising power, however, decreases in the same order — HClO is the strongest oxidising agent among chlorine oxoacids, not HClO₄. Students confuse acid strength (increases with oxidation state) with oxidising power (decreases with oxidation state).
Getting XeF₄ Shape Wrong
XeF₄ is square planar (not tetrahedral). Xe has 6 electron pairs total (4 bond + 2 lone). The lone pairs go axial in an octahedral arrangement, leaving the 4 F atoms in the equatorial plane → square planar. Students who use the number of atoms (4) instead of total electron pairs get "tetrahedral" and lose the mark.
Your p-Block Revision Checklist
- State 4 ways boron differs from aluminium (size, covalent vs ionic, Lewis acidity, max covalency).
- Explain why boric acid is a Lewis acid, not a Brønsted acid.
- State why NCl₅ does not exist but PCl₅ does.
- Name and give the oxidation state of nitrogen in all 3 main oxoacids (HNO₂, HNO₃).
- Determine the basicity of any phosphorus oxoacid by counting P–OH bonds only.
- State the oxidising power trend of halogens and explain why F is the strongest.
- Explain why HF is a weak acid while HCl is strong.
- Identify the acid strength and thermal stability order of chlorine oxoacids.
- Draw and name the structures of XeF₂, XeF₄, XeF₆ using VSEPR.
- State the bleaching mechanism of SO₂ (reduction) vs Cl₂ (oxidation).
p-Block Elements rewards students who understand trends and anomalies, not those who memorise isolated facts. Every anomaly has a reason — small size, absence of d-orbitals, high electronegativity, inert pair effect — and knowing the reason lets you answer questions about compounds you have never seen before. That is the difference between 3 marks and 8 marks from this chapter in NEET.
For Inorganic context, the Coordination Compounds guide covers d-block chemistry with a similar focus on trends and structures. If you want to work through specific p-block reaction questions or oxoacid structures with immediate feedback, book a free 30-minute demo class. Bring the subtopic you find hardest — we will work through the logic together.