The p-Block Elements (Groups 15 to 18) covers the trends, oxidation states and key compounds of the nitrogen family, the oxygen family, the halogens and the noble gases. It includes the preparation and properties of ammonia, nitric acid, sulphuric acid, ozone, interhalogen compounds and xenon fluorides. This is a high-yield NEET chapter where questions test group trends, anomalous behaviour of the first element, industrial processes, and the structures of compounds like the xenon fluorides.
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The p-Block at a Glance
Groups 15-18 fill the np orbitals, and each group’s top element behaves oddly because period-2 atoms have no d-orbitals.
Down a group: metallic character rises, non-metallic falls.
- First member (N, O, F, Ne) is anomalous: small size, high EN, no d-orbitals
- Inert pair effect makes lower oxidation states more stable down a group
- Maximum covalency of period-2 elements is limited (N=4, O=2, F=1)
Nitrogen Family Trends
The nitrogen family (N, P, As, Sb, Bi) shows oxidation states −3, +3, +5 with predictable stability shifts down the group.
N≡N is a gas; P, As exist as tetrahedral P4 units.
- Hydride bond angle: NH3 (107°) > PH3 (~93°) > AsH3 > SbH3
- Hydride basicity & thermal stability ↓, reducing power ↑ down the group
- N forms pπ–pπ multiple bonds; no NCl5 (no d-orbitals), but PCl5 exists
NH3, HNO3 & N-Oxides
Ammonia and nitric acid are the key industrial Group-15 compounds, with nitrogen oxides spanning +1 to +5.
HNO3 (Ostwald, Pt/Rh): conc.→NO2, dilute→NO, never H2.
- NH3 is a pyramidal Lewis base; gives deep blue [Cu(NH3)4]2⁺
- Oxides: N2O (+1), NO (+2, paramagnetic), NO2 (+4, brown), N2O5 (+5)
- Aqua regia = 3 HCl : 1 HNO3 dissolves Au and Pt
Phosphorus & Its Oxoacids
Phosphorus has reactive white and stable red allotropes, and its oxoacids differ by the number of P–OH groups.
P–H bonds give reducing power; H3PO4 (0 P–H) is not reducing.
- White P4 is reactive, glows, stored under water; red P is polymeric and stable
- H3PO2 (+1, 2 P–H) is a strong reducing agent, H3PO3 (+3, 1 P–H)
- PH3 from P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2; weaker base than NH3
Oxygen Family (Chalcogens)
The chalcogens (O, S, Se, Te, Po) sit two electrons short of an octet and favour the −2 state.
O exceptions: +2 in OF2, −1 in peroxides; max covalency of O = 2.
- Hydride acidity ↑ down: H2O < H2S < H2Se < H2Te
- H2O is liquid (H-bonding); bond angle H2O (104.5°) > H2S > H2Se
- Ozone O3 is angular (~117°), a powerful oxidiser: 2O3 → 3O2
SO2 & Sulphuric Acid
Sulphuric acid is made by the Contact process and acts as acid, dehydrating agent and oxidiser.
In H2SO4, S is sp3 and +6; SO2 bleaching is temporary (by reduction).
- Contact process: burn S → SO2, oxidise to SO3 over V2O5 catalyst
- SO2 is acidic and reducing; its bleaching reverses on exposure to air
- Oleum is diluted with water to give concentrated H2SO4
Halogens & Fluorine Anomaly
The halogens (F, Cl, Br, I, At) are the most electronegative group and the strongest oxidisers.
Electron gain enthalpy: Cl > F (F small → strong e⁻–e⁻ repulsion).
- F shows only −1 (no d-orbitals); Cl, Br, I reach +1, +3, +5, +7
- HF is liquid (H-bonding) and a weak acid; acidity HF < HCl < HBr < HI
- HCl prep: NaCl + H2SO4 → NaHSO4 + HCl
Interhalogens & Oxoacids
Halogens combine with each other and with oxygen to give reactive interhalogens and a ladder of oxoacids.
Interhalogens (XX’ₙ) are more reactive than the parent halogens.
- ClF3 is T-shaped, IF5 square pyramidal, IF7 pentagonal bipyramidal
- Acidity and oxidation state of Cl oxoacids rise from HOCl to HClO4
- HOCl is the strongest oxidiser, HClO4 the strongest acid
Noble Gases & Xenon Compounds
Noble gases have a completely filled shell making them inert, yet xenon’s low ionisation enthalpy lets it react with F and O.
Use VSEPR (count lone pairs) to get xenon-compound geometry.
- Monatomic, largest (van der Waals) radii in their period, low boiling points
- Xe reacts because it has the lowest ionisation enthalpy among stable noble gases
- XeO3 pyramidal, XeOF4 square pyramidal, XeO4 tetrahedral
📝 Practice The p-Block Elements (Groups 15-18) — 10 NEET PYQs
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Related Chapters in Class 12 Chemistry
- The d and f Block Elements Class 12 Notes
- General Principles and Processes of Isolation of Elements Class 12 Notes
- Coordination Compounds Class 12 Notes
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Frequently Asked Questions
The inert pair effect is the reluctance of the outermost s-electrons to take part in bonding as we move down a group. In Group 15 it makes the +5 oxidation state less stable and the +3 state more stable down the group, so bismuth is most stable in the +3 state.
Ammonia is made by Haber’s process, the reaction of nitrogen with hydrogen over an iron catalyst at about 200 atmospheres and 700 K. Nitric acid is made by Ostwald’s process, the catalytic oxidation of ammonia over a platinum-rhodium catalyst followed by further oxidation and absorption in water.
Fluorine is the most electronegative element and has no available d-orbitals in its valence shell, so it cannot expand its octet. As a result it shows only the minus one oxidation state, whereas the other halogens (chlorine, bromine, iodine) can show positive states such as +1, +3, +5 and +7.
Xenon difluoride (XeF2) is linear, xenon tetrafluoride (XeF4) is square planar, and xenon hexafluoride (XeF6) is a distorted octahedron. Noble gases react only with the most electronegative elements fluorine and oxygen, and xenon does so because it has the lowest ionisation energy among the stable noble gases.
Oxidising power depends on the ease of accepting an electron and the overall energetics of reduction. Fluorine is the strongest oxidiser because of its low bond dissociation enthalpy and the high hydration enthalpy of the fluoride ion, even though its electron gain enthalpy is anomalously lower than chlorine. The power then decreases down the group to iodine.