Iodine (I): Comprehensive Study Guide

Introduction

Iodine (I) is a non-metallic element belonging to Group 17 (halogens) of the periodic table. It is the least reactive of the common halogens and the most electropositive. In its elemental form, iodine exists as a diatomic molecule, I₂. Iodine’s significance extends beyond theoretical chemistry, playing vital roles in human health, industrial processes, and analytical applications.

CBSE/JEE Quick Revision Notes

• Symbol: I
• Atomic Number: 53
• Atomic Mass: 126.90 u
• Group: 17 (Halogens)
• Period: 5
• Block: p-block
• Electronic Configuration: [Kr] 4d¹⁰ 5s² 5p⁵
• Valency: -1 (most common in ionic compounds); can exhibit +1, +3, +5, +7 in covalent compounds.
• Physical State at RTP: Solid (purplish-black crystalline solid)
• Nature: Non-metal, halogen. Sublimes readily to form a violet vapor.
• Electronegativity (Pauling): 2.66
• Ionization Enthalpy (first): 1008.4 kJ/mol
• Electron Gain Enthalpy: -295.2 kJ/mol (less negative than F, Cl, Br)
• Oxidizing Nature: Weaker oxidizing agent than F₂, Cl₂, and Br₂.

Electron Configuration & Bonding Behavior

Iodine’s electron configuration, [Kr] 4d¹⁰ 5s² 5p⁵, indicates it has seven valence electrons. To achieve a stable octet, it readily gains one electron, forming the iodide ion (I⁻) with an oxidation state of -1.

However, due to the presence of vacant d-orbitals in its valence shell, iodine can expand its octet and exhibit higher positive oxidation states (+1, +3, +5, +7) by promoting electrons. This is observed in interhalogen compounds (e.g., IF₃, IF₅, IF₇) and oxoacids (e.g., HIO, HIO₃, HIO₄) and their salts.

Iodine forms:

• Ionic bonds with highly electropositive metals (e.g., NaI, KI).
• Covalent bonds with non-metals and less electropositive elements (e.g., HI, ICl).

Crucial Chemical Reactions

1. Reaction with Metals

Iodine reacts with many metals, often requiring heating, to form metal iodides.

• With Sodium: 2Na(s) + I₂(s) → 2NaI(s)

• With Iron: Fe(s) + I₂(s) → FeI₂(s)

2. Reaction with Hydrogen

Hydrogen and iodine react reversibly to form hydrogen iodide (HI). The reaction is less vigorous and less exothermic than with other halogens.

• H₂(g) + I₂(s) ⇌ 2HI(g)

3. Reaction with Water

Iodine is sparingly soluble in water and does not react to the same extent as chlorine or bromine. It disproportionates slowly, especially in the presence of sunlight.

• I₂(s) + H₂O(l) ⇌ HI(aq) + HIO(aq)

4. Displacement Reactions (Halide Oxidation)

Iodine (as I⁻) can be displaced from its halide solutions by stronger oxidizing halogens like chlorine and bromine.

• By Chlorine: 2NaI(aq) + Cl₂(g) → 2NaCl(aq) + I₂(aq)

• By Bromine: 2NaI(aq) + Br₂(l) → 2NaBr(aq) + I₂(aq)

Note: Iodine cannot displace chlorine or bromine from their respective halide solutions.

5. Reaction with Oxidizing Agents

Iodine can be oxidized to higher oxidation states by strong oxidizing agents.

• With Concentrated Nitric Acid: I₂(s) + 10HNO₃(conc) → 2HIO₃(aq) + 10NO₂(g) + 4H₂O(l)

• With Concentrated Sulfuric Acid: I₂(s) + 10H₂SO₄(conc) → 2HIO₃(aq) + 9SO₂(g) + 4H₂O(l)

6. Reaction with Thiosulphate (Iodometry/Iodimetry)

This reaction is fundamental in quantitative analysis, particularly in iodometric titrations.

• I₂(aq) + 2S₂O₃²⁻(aq) → 2I⁻(aq) + S₄O₆²⁻(aq) (Iodine is reduced to iodide, thiosulphate is oxidized to tetrathionate.)

7. Test for Iodine (Starch Test)

Iodine forms a characteristic blue-black complex with starch solution. This is a highly sensitive test for the presence of free iodine.

• I₂(aq) + Starch Solution → Blue-Black Complex (The color disappears on heating and reappears on cooling, indicating a physical interaction rather than a chemical reaction.)

Industrial and Biological Importance

Industrial Importance

• Antiseptics and Disinfectants: Tincture of iodine (2-3% I₂ solution in alcohol/water) is an effective antiseptic. Povidone-iodine is widely used in medical settings.
• Catalyst: Iodine compounds are used as catalysts in certain organic reactions.
• Analytical Reagent: Used extensively in iodometry and iodimetry for the quantitative determination of oxidizing and reducing agents.
• Photography: Silver iodide (AgI) is used in photographic emulsions.
• Artificial Rain: Silver iodide crystals are used in cloud seeding to induce rainfall.
• Polarizers: Poly(vinyl alcohol) sheets containing iodine are used in polarizing filters.

Biological Importance

• Thyroid Hormones: Iodine is an essential trace element required for the synthesis of thyroid hormones (Thyroxine, T₄ and Triiodothyronine, T₃) by the thyroid gland. These hormones regulate metabolism, growth, and development.
• Iodine Deficiency Disorders (IDDs): Insufficient iodine intake can lead to:
  • Goitre: Enlargement of the thyroid gland.
  • Cretinism: Severe physical and mental retardation in children, caused by congenital iodine deficiency.
  • Other metabolic and developmental issues.
• Dietary Sources: Iodized salt is the primary source of iodine for many populations. Seafood, dairy products, and certain vegetables also contain iodine.
• Contrast Agents: Some iodine-containing organic compounds are used as X-ray contrast agents in medical imaging.