Inorganic ChemistryTop Priority (3-4 Qs in JEE)

p-Block Elements Formulas

Group 15 (N, P, As), Group 16 (O, S), Group 17 (Halogens), Group 18 (Noble Gases) — oxoacids, interhalogen compounds, allotropes, and trend rules.

9 Formulas•894+ Questions in Bank

Core Formulas & Mathematical Identities

Oxidation States of Group 15, 16, 17 Elements

\text{Group 15: } -3, +1, +2, +3, +4, +5 \quad \text{Group 16: } -2, +2, +4, +6 \quad \text{Group 17: } -1, +1, +3, +5, +7

p-Block elements exhibit multiple oxidation states due to availability of d-orbitals (from 3rd period onward). Fluorine has fixed OS = −1 (no d-orbital, most electronegative).

Examiner Tip: Nitrogen does NOT show +5 OS in fluoride (NF₅ does not exist) because N has no d-orbitals to expand octet. Maximum OS of N is +4 in NO₂ (and +5 only in N₂O₅ / NO₃⁻).

Phosphorus Oxoacids — Basicity & Reducing Power

\begin{array}{lcc} \text{Acid} & \text{Basicity} & \text{Reducing?} \\ H_3PO_4 & 3 & \text{No} \\ H_3PO_3 & 2 & \text{Yes} \\ H_3PO_2 & 1 & \text{Yes (strong)} \\ H_4P_2O_7 & 4 & \text{No} \end{array}

Basicity = number of ionisable P−OH groups. P−H bonds are not ionisable. Reducing power arises from P−H bond which donates H to oxidising agent.

Examiner Tip: Memory rule: Count only OH groups directly bonded to P. H₃PO₂ has one −OH and two P−H bonds → monobasic, strong reducing agent. All metaphosphoric acids (HPO₃)ₙ have basicity 1.

Nitrogen Oxides Oxidation State Ladder

N_2O \xrightarrow{+1} NO \xrightarrow{+2} N_2O_3 \xrightarrow{+3} NO_2 \xrightarrow{+4} N_2O_4 \xrightarrow{+4} N_2O_5 \xrightarrow{+5}

Nitrogen oxides in increasing oxidation state. NO and NO₂ are paramagnetic (odd-electron species). N₂O₄ is the dimer of NO₂ (diamagnetic).

Examiner Tip: Only NO₂ (brown gas) and N₂O₄ (colourless) are in equilibrium: 2NO₂ ⇌ N₂O₄ (K_p decreases with temperature → more NO₂ at high T, Le Chatelier). N₂O is laughing gas (+1 OS), used as anaesthetic.

Oxoacids of Sulphur — Structure & OS

H_2S_2O_7 \text{ (pyrosulphuric, oleum, OS +7)}, \quad H_2SO_4 \text{ (+6)}, \quad H_2SO_3 \text{ (+4)}, \quad H_2S_2O_8 \text{ (peroxodisulphuric, Marshall's acid, OS +6)}

Key sulphur oxoacids with S oxidation states. Oleum (H₂S₂O₇) = SO₃ dissolved in H₂SO₄. Marshall's acid contains -O-O- peroxo linkage. Thiosulphate Na₂S₂O₃ contains S in OS 0 and +5 (average +2).

Examiner Tip: H₂S₂O₃ (thiosulphuric acid): average OS of S = +2 but one S is -2 and other is +6 (non-equivalent). Hot conc. H₂SO₄ acts as oxidising agent (reduces to SO₂); cold dil. H₂SO₄ acts as acid.

Interhalogen Compounds — Formula & Geometry

\text{Type XY: ClF, BrF, BrCl} \quad \text{XY}_3\text{: ClF}_3 \text{ (T-shape)} \quad \text{XY}_5\text{: ClF}_5\text{, BrF}_5 \text{ (Square pyramidal)} \quad \text{XY}_7\text{: IF}_7 \text{ (Pentagonal bipyramidal)}

Interhalogen compounds XYₙ where X is the heavier halogen and n = 1, 3, 5, 7. Geometry determined by VSEPR including lone pairs.

Examiner Tip: ClF₃ has 2 lone pairs on Cl → T-shape (sp³d). BrF₅ has 1 lone pair on Br → square pyramidal (sp³d²). IF₇ has 0 lone pairs on I → pentagonal bipyramidal (sp³d³). All are powerful fluorinating agents.

Noble Gas Compounds of Xenon — Structure & Shape

\begin{array}{lll} \text{Compound} & \text{Hybridisation} & \text{Shape} \\ XeF_2 & sp^3d & \text{Linear} \\ XeF_4 & sp^3d^2 & \text{Square planar} \\ XeF_6 & sp^3d^3 & \text{Distorted octahedral} \\ XeO_3 & sp^3 & \text{Pyramidal} \\ XeO_4 & sp^3 & \text{Tetrahedral} \end{array}

Xenon fluorides and oxides. XeF₂ and XeF₄ have lone pairs on Xe making them linear and square planar respectively (VSEPR).

Examiner Tip: XeF₂ is a mild fluorinating agent. XeF₄ reacts with water to give Xe + O₂ + HF (disproportionation): 6XeF₄ + 12H₂O → 4Xe + 2XeO₃ + 24HF + 3O₂. Never confuse XeF₄ (sp³d²) with SF₄ (sp³d, seesaw).

Disproportionation of Halogen Acids & Oxyhalogen Acids

\text{Hot NaOH: } 3Cl_2 + 6NaOH \xrightarrow{\Delta} 5NaCl + NaClO_3 + 3H_2O

Disproportionation of halogens in alkali. In cold dilute NaOH: Cl₂ + 2NaOH → NaCl + NaOCl + H₂O (hypohalite formation). Bleaching powder: Ca(OCl)Cl formed by Cl₂ + Ca(OH)₂ → Ca(OCl)Cl + H₂O.

Examiner Tip: Oxidising power order of oxyacids: HOCl > HOBr > HOI. Acid strength order (opposite to oxidising power): HClO₄ > HClO₃ > HClO₂ > HOCl. HOCl is the strongest oxidising halogen oxyacid.

Allotropes of Phosphorus & Sulphur — Key Properties

\text{P}_4 \text{ (white, P−P−P angle } 60°, \text{most reactive)}, \quad \text{Red P} \text{ (polymer)}, \quad \text{Black P} \text{ (graphite-like, most stable)}

Sulphur allotropes: rhombic S₈ (stable below 96°C), monoclinic S₈ (stable 96–119°C), plastic S (amorphous, above 160°C). White phosphorus is soluble in CS₂ and is poisonous; red phosphorus is non-toxic polymer.

Examiner Tip: White P₄ has tetrahedral structure with bond angle 60° (highly strained → very reactive). Reacts with conc. NaOH: P₄ + 3NaOH + 3H₂O → 3NaH₂PO₂ + PH₃↑ (disproportionation). Contact process for H₂SO₄: 2SO₂ + O₂ ⇌ 2SO₃ (V₂O₅ catalyst, 450°C, 2 atm).

Acidic Strength & Reducing Power Trends in Group 16, 17

\text{Acid strength: HF} < \text{HCl} < \text{HBr} < \text{HI (bond energy ↓)} \quad \text{Reducing power: HF} < \text{HCl} < \text{HBr} < \text{HI}

Hydrohalic acid strength increases down the group as H−X bond dissociation enthalpy decreases. HF is the weakest acid but forms strongest H-bonds. HF etches glass: SiO₂ + 4HF → SiF₄ + 2H₂O.

Examiner Tip: Bond angle in H₂O > H₂S > H₂Se > H₂Te (O uses pure p-orbitals; as we go down, angles → 90° as pure p-orbitals used for bonding). Thermal stability of hydrides: H₂O > H₂S > H₂Se > H₂Te (bond energy ↓ down group).

Top Examiner Traps & Pitfalls

Frequently tested misconceptions in p-Block Elements

•PCl₅ hybridisation is sp³d (NOT sp³d²). Hybridisation of XeF₄ is sp³d² (NOT sp³d).
•H₃PO₃ is diprotic (only 2 OH groups) and H₃PO₂ is monoprotic (only 1 OH group) — P=O and P-H bonds are non-ionisable.

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