Learning Objectives
- Study Group 15 (Nitrogen family), Group 16 (Oxygen family), Group 17 (Halogens), and Group 18 (Noble gases)
- Learn about important compounds: ammonia, nitric acid, sulphuric acid, ozone
- Understand trends in properties and anomalous behaviour of first element
- Study interhalogen compounds and oxoacids
- Know about noble gas compounds and their bonding
Key Concepts
Group 15 (N, P, As, Sb, Bi)
Configuration: ns²np³. Oxidation states: -3, +3, +5. Stability of +3 increases down group (inert pair effect).
N₂ has a triple bond (very stable, high bond energy 941 kJ/mol). Nitrogen shows anomalous behaviour due to small size, high EN, and absence of d-orbitals.
Ammonia (NH₃): Haber process: N₂ + 3H₂ ⇌ 2NH₃ (Fe catalyst, 200 atm, 700K). Pyramidal (sp³). Basic, forms NH₄⁺. Gives white fumes with HCl.
Nitric acid (HNO₃): Ostwald process: 4NH₃ + 5O₂ → 4NO + 6H₂O (Pt catalyst); 2NO + O₂ → 2NO₂; 3NO₂ + H₂O → 2HNO₃ + NO. Strong oxidising acid.
Oxides of nitrogen: N₂O (neutral), NO (neutral), N₂O₃ (acidic), NO₂ (acidic), N₂O₅ (acidic).
Group 16 (O, S, Se, Te, Po)
Configuration: ns²np⁴. Oxidation states: -2, +2, +4, +6. Oxygen shows -2 mostly (no d-orbitals).
Ozone (O₃): Bent structure, resonance hybrid. Powerful oxidising agent. Decomposes to O₂.
Sulphuric acid (H₂SO₄): Contact process: S + O₂ → SO₂; 2SO₂ + O₂ → 2SO₃ (V₂O₅, 720K); SO₃ + H₂SO₄ → H₂S₂O₇ (oleum); H₂S₂O₇ + H₂O → 2H₂SO₄.
Properties: Strong diprotic acid, dehydrating agent, oxidising agent. Reacts with metals, non-metals, compounds.
Group 17 (Halogens: F, Cl, Br, I, At)
Configuration: ns²np⁵. Most reactive non-metals. Oxidation states: -1 (all), +1, +3, +5, +7 (Cl, Br, I).
Reactivity: F₂ > Cl₂ > Br₂ > I₂. Electron gain enthalpy: Cl > F > Br > I (F has smaller size).
Anomalous properties of F: Small size, high EN (4.0), no d-orbitals, forms only -1 state, strongest oxidising agent.
HF: Weakest hydrohalic acid (strong H-bonding). HI: strongest acid (weakest H-I bond).
Acid strength: HF < HCl < HBr < HI. Bond energy: H-F > H-Cl > H-Br > H-I.
Oxoacids of chlorine: HOCl (hypochlorous) < HClO₂ (chlorous) < HClO₃ (chloric) < HClO₄ (perchloric). Acid strength increases with oxidation state of Cl.
Interhalogen compounds: XX' (ClF), XX'₃ (ClF₃, BrF₃), XX'₅ (BrF₅, IF₅), XX'₇ (IF₇). More reactive than parent halogens.
Group 18 (Noble Gases: He, Ne, Ar, Kr, Xe, Rn)
Configuration: ns²np⁶ (except He: 1s²). Complete octet → very stable, low reactivity.
Xenon compounds: XeF₂ (linear, sp³d), XeF₄ (square planar, sp³d²), XeF₆ (distorted octahedral, sp³d³). XeO₃ (pyramidal), XeOF₂ (T-shaped).
Prepared by direct reaction of Xe with F₂ at different ratios and conditions.
Summary
Groups 15-18 show diverse chemistry. Group 15: nitrogen forms strong triple bonds; ammonia and nitric acid are key compounds. Group 16: oxygen and sulphur show varied oxidation states; H₂SO₄ is the most important industrial chemical. Group 17: halogens are the most reactive non-metals; form oxoacids and interhalogen compounds. Group 18: noble gases are mostly inert, but Xe forms fluorides and oxides.
Important Terms
- Inert Pair Effect: Stability of +3 over +5 state in heavier elements
- Allotropy: S₈ (rhombic, monoclinic sulphur), O₂, O₃
- Interhalogen: Compounds of two different halogens
- Oxoacid: Acid containing oxygen, hydrogen, and another element
- Disproportionation: Halogen in alkali → halide + hypohalite
Quick Revision
- NH₃: sp³, Haber process (Fe, 200 atm); HNO₃: Ostwald (Pt)
- H₂SO₄: Contact process (V₂O₅); oleum = H₂S₂O₇
- Acid strength of HX: HF < HCl < HBr < HI
- Oxoacid strength: HOCl < HClO₂ < HClO₃ < HClO₄
- XeF₂: linear; XeF₄: square planar; XeF₆: distorted octahedral
- F has no +ve oxidation state; strongest oxidising halogen