Group 17: properties, reactions, trends, uses

🔬 What Are the Halogens?

Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and the rare astatine (At) form the halogen family. Think of them as a group of very reactive cousins who love to form bonds with almost everyone else.

⚡️ Key Properties

• Highly electronegative: F has the highest electronegativity ($\chi_{\text{F}} = 3.98$).
• Strong oxidising agents: they readily accept electrons from metals.
• State at room temperature: F and Cl are gases, Br is a liquid, I is a solid.
• Colourful: Cl₂ is greenish‑yellow, Br₂ is deep red, I₂ is violet.
• High ionisation energies decrease down the group.

🧪 Common Reactions

1. With metals: $M + X_2 \rightarrow M^{2+} + 2X^-$ (e.g., $\text{Na} + \text{Cl}_2 \rightarrow \text{NaCl}$).
2. With hydrogen: $X_2 + H_2 \rightarrow 2HX$ (e.g., $\text{Cl}_2 + \text{H}_2 \rightarrow 2HCl$).
3. With water: $X_2 + H_2O \rightarrow HX + HOX$ (hydrolysis, e.g., $\text{Cl}_2 + H_2O \rightarrow HCl + HOCl$).
4. With oxygen: $2X_2 + O_2 \rightarrow 2XO$ (e.g., $\text{Cl}_2 + O_2 \rightarrow 2ClO$).
5. With organic compounds: halogenation of alkanes (free‑radical substitution).

📈 Trend Chart

🌍 Everyday Uses

• Chlorine: disinfecting swimming pools and drinking water.
• Fluorine: in toothpaste (fluorides) to prevent cavities.
• Bromine: fire‑retardant foams and in photographic chemicals.
• Iodine: antiseptic solutions and in thyroid medication.
• Astatine: research in nuclear medicine (rare).

💡 Exam Tips

When answering questions about halogens:

• Show the trend: electronegativity ↓, atomic radius ↑, reactivity ↓ down the group.
• Remember the oxidation states: common +1, +3, +5, +7 (except F which is always –1).
• Use the halogenation reaction as a template: $R–H + X_2 \xrightarrow{\text{radical}} R–X + HX$.
• Highlight the real‑world applications to show relevance.