p-Block Elements (Groups 15-18) — chemistry Class 12 Notes (CBSE & HBSE)
Free NCERT chemistry notes for p-Block Elements (Groups 15-18) (Class 12) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — p-Block Elements (Groups 15-18) (CBSE & HBSE)
Group 15 (N, P): allotropes, compounds. Group 16 (O, S): allotropes, H2SO4. Group 17 halogens and Group 18 noble gases, interhalogen compounds.
Group 15 Elements - Nitrogen and Phosphorus
p-Block Elements (Groups 15-18)
Group 15 - General Properties
Elements: N, P, As, Sb, Bi Electronic config: ns^2 np^3 (half-filled, stable)
| Property | N | P | As | Sb | Bi |
|---|---|---|---|---|---|
| Atomic no. | 7 | 15 | 33 | 51 | 83 |
| Oxidation states | -3 to +5 | -3 to +5 | -3 to +5 | -3 to +5 | -3 to +5 |
| Metallic character | Increases down the group |
Dinitrogen (N2)
- Very stable: N≡N bond (triple bond, 941 kJ/mol)
- Inert at room temperature (kinetic stability)
- 78% of atmosphere
- Preparation: NH4NO2 → N2 + 2H2O (laboratory)
- Commercially: fractional distillation of liquid air
Ammonia (NH3)
Industrial: Haber process: N2 + 3H2 → 2NH3 (Fe catalyst, 500 C, 200 atm) Laboratory: Ca(OH)2 + 2NH4Cl → CaCl2 + 2NH3 + 2H2O
Properties:
- Pyramidal molecule (sp3, lone pair present)
- Strong H-bonds; high bp (-33.4 C)
- Weak base: NH3 + H2O ⇌ NH4+ + OH-
Oxides of Nitrogen
| Oxide | Name | Oxidation state of N | Character |
|---|---|---|---|
| N2O | Nitrous oxide (laughing gas) | +1 | Neutral |
| NO | Nitric oxide | +2 | Neutral (odd e-) |
| N2O3 | Dinitrogen trioxide | +3 | Acidic |
| NO2 | Nitrogen dioxide | +4 | Acidic (odd e-) |
| N2O5 | Dinitrogen pentoxide | +5 | Acidic (anhydride of HNO3) |
Nitric Acid (HNO3)
Industrial: Ostwald Process 4NH3 + 5O2 (Pt/Rh, 800 C) → 4NO + 6H2O 2NO + O2 → 2NO2 3NO2 + H2O → 2HNO3 + NO
Properties:
- Strong acid (monoprotic)
- Oxidizing agent (conc. and dilute)
- Dilute HNO3 + metals → metal nitrate + H2O + NO (gas)
- Conc. HNO3 + Cu → Cu(NO3)2 + NO2 (brown gas) + H2O
- Conc. HNO3 passivates Fe and Al (protective oxide film)
Phosphorus Allotropes
White P (P4): Tetrahedral P4 molecule; very reactive; toxic; stored in water. Red P: Polymeric; less reactive; non-toxic; used in matchboxes. Black P: Most stable; layered; good conductor.
Phosphine (PH3)
Preparation: P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2 Trigonal pyramidal; weak base; reduces to P in air.
Group 16 Elements - Oxygen and Sulfur
Group 16 Elements
Elements: O, S, Se, Te, Po Configuration: ns^2 np^4; oxidation states -2, -1, +2, +4, +6 (except O: -2 and -1)
Dioxygen (O2)
Diatomic; paramagnetic (2 unpaired electrons in antibonding pi* orbitals). Preparation: 2KClO3 (MnO2, 300 C) → 2KCl + 3O2 2H2O2 (MnO2) → 2H2O + O2
Ozone (O3)
Allotrope of oxygen; pale blue gas; pungent smell. Preparation: O2 + silent electrical discharge (ozonizer) → O3 (6-16% ozone) Structure: Bent, V-shaped; resonance (O-O=O ↔ O=O-O); bond angle 117 C Ozone layer: Absorbs UV-C radiation (200-300 nm) in stratosphere.
Properties:
- Powerful oxidizing agent (O3 → O2 + O, nascent oxygen)
- O3 + 2KI + H2O → I2 + 2KOH + O2 (I2 turns starch blue; test for ozone)
- Bleaching: O3 bleaches many dyes by oxidation
Allotropes of Sulfur
Rhombic Sulfur (alpha-S): Stable below 96 C; yellow crystals; puckered S8 rings. Monoclinic Sulfur (beta-S): Stable 96-119 C; also S8 rings but different packing. Transition temperature (96 C): alpha ↔ beta interconversion.
Sulfur Dioxide (SO2)
Preparation: S + O2 → SO2 (burning sulfur) 2ZnS + 3O2 → 2ZnO + 2SO2 (roasting)
Properties:
- Acidic gas; SO2 + H2O → H2SO3
- Reducing agent (in acid medium)
- Bleaching agent (used to bleach wool, silk by reduction)
- Air pollutant; causes acid rain: SO2 + H2O → H2SO3; SO3 + H2O → H2SO4
Sulfuric Acid (H2SO4) - Contact Process
Step 1: S + O2 → SO2 (burning S or roasting FeS2) Step 2: 2SO2 + O2 → 2SO3 (V2O5 catalyst, 450 C, 1 atm) Step 3: SO3 + H2SO4 → H2S2O7 (oleum) Step 4: H2S2O7 + H2O → 2H2SO4
(SO3 is not dissolved directly in water as it forms mist)
Properties of H2SO4:
- Strong diprotic acid
- Dehydrating agent: absorbs water from CuSO4.5H2O (blue → white), sucrose → carbon
- Oxidizing agent (hot conc.): Cu + 2H2SO4(hot conc.) → CuSO4 + SO2 + 2H2O
Group 17 (Halogens) and Group 18 (Noble Gases)
Group 17 - Halogens
Elements: F, Cl, Br, I, At Configuration: ns^2 np^5; most electronegative, highest oxidizing power in their period. Oxidation states: -1 (most common), +1, +3, +5, +7 (except F: only -1 and 0) F has no d-orbitals; cannot expand valence shell; only -1 state.
General Properties
| Property | F | Cl | Br | I |
|---|---|---|---|---|
| State | Gas | Gas | Liquid | Solid |
| Color | Pale yellow | Yellow-green | Red-brown | Violet-black |
| Oxidizing power | Decreases down group (F is strongest) | |||
| Bond energy | F-F weakest | Cl-Cl moderate | Br-Br | I-I weakest |
| Electronegativity | Highest (4.0) | 3.0 | 2.8 | 2.5 |
Anomalous Behavior of Fluorine
- Smallest size, highest electronegativity (4.0)
- No d-orbitals → no expanded valence; maximum oxidation state = +1 (actually only -1)
- HF is a weak acid (due to strong H-F bond); HCl, HBr, HI are strong acids
- F-F bond is unusually weak (155 kJ/mol) due to lone pair-lone pair repulsion in small F atoms
- F forms H-bonds (HF is liquid, others are gases at RT)
- Only F has +2 state in OF2 (oxygen is +2; F has -1)
Chlorine Preparation
Laboratory: MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O Industrial: Chlor-alkali process (electrolysis of brine): Cathode: 2H2O + 2e- → H2 + 2OH-; Anode: 2Cl- → Cl2 + 2e-
Properties of Cl2:
- Yellow-green, pungent; oxidizing and bleaching agent
- Cl2 + H2O ⇌ HOCl + HCl (bleaching due to HOCl and nascent O)
- Cl2 + 2NaOH → NaCl + NaOCl + H2O (sodium hypochlorite = bleaching powder)
- 2Cl2 + 2Ca(OH)2 → CaCl2 + Ca(OCl)2 + 2H2O (bleaching powder)
Interhalogen Compounds
Formed between two different halogens: AB, AB3, AB5, AB7 Examples: ClF, BrF3, IF5, IF7, ClF3, BrF5 More reactive than halogens; good fluorinating agents. Bond: More electronegative halogen is at terminal position.
Group 18 - Noble Gases
Elements: He, Ne, Ar, Kr, Xe, Rn Full s and p subshells (He: 1s^2; others: ns^2 np^6) Extremely low reactivity; very high ionization energies.
Compounds of Xenon
XeF2 (linear), XeF4 (square planar), XeF6 (distorted octahedral) XeO3 (pyramidal, strong oxidizing agent)
Prepared by: Xe + F2 → XeF2/XeF4/XeF6 (varying ratios at different conditions)
Uses of Noble Gases
| Gas | Uses |
|---|---|
| He | Balloons (non-flammable), deep-sea diving, MRI, cooling for superconductors |
| Ne | Advertising signs (red/orange glow) |
| Ar | Incandescent bulbs (inert atmosphere), arc welding |
| Kr | High-intensity lights |
| Xe | Flashlights, NMR, anesthetic |
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