Halogens: Properties, Trends & Uses

TAKEAWAYS:

• Halogens are Group 17 elements (F₂, Cl₂, Br₂, I₂, At) with 7 valence electrons, forming -1 ions and covalent bonds in compounds.
• Their reactivity decreases down the group (F₂ > Cl₂ > Br₂ > I₂) due to atomic size and bond dissociation energy.
• Disproportionation (e.g., Cl₂ + H₂O → HCl + HClO) and oxidizing power are key reactions.
• Uses: Chlorine in water purification, iodine in antiseptics, and fluorine in Teflon.
• Environmental impact: Ozone depletion by CFCs (chlorofluorocarbons) and toxic halogen gases.

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### **1. Introduction to Halogens**
Halogens are **Group 17** elements in the periodic table. They are **highly reactive non-metals** and include:
- **Fluorine (F₂)**
- **Chlorine (Cl₂)**
- **Bromine (Br₂)**
- **Iodine (I₂)**
- **Astatine (At)** (radioactive, rare)

They are called **"salt-formers"** because they react with metals to form **salts** (e.g., NaCl, KCl).

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### **2. Physical Properties of Halogens**
| **Property**       | **Fluorine (F₂)**       | **Chlorine (Cl₂)**      | **Bromine (Br₂)**       | **Iodine (I₂)**        |
|--------------------|--------------------------|-------------------------|-------------------------|------------------------|
| **State at RT**    | Pale yellow **gas**      | Greenish-yellow **gas** | Reddish-brown **liquid**| Purple-black **solid** |
| **Melting Point**  | -220°C                   | -101°C                  | -7°C                    | 114°C                  |
| **Boiling Point**  | -188°C                   | -34°C                   | 59°C                    | 184°C                  |
| **Density**        | 1.69 g/L (gas)           | 3.21 g/L (gas)          | 3.10 g/mL (liquid)      | 4.93 g/cm³ (solid)     |
| **Color**          | Pale yellow              | Greenish-yellow         | Reddish-brown           | Shiny purple-black     |

```figure
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Trend: As we go down the group, melting and boiling points increase due to increasing van der Waals forces between larger molecules.

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3. Chemical Properties of Halogens

A. Reactivity

• Halogens are highly reactive because they need only 1 electron to complete their octet.
• Reactivity decreases down the group because:
  • Atomic size increases → nuclear attraction decreases → harder to gain an electron.
  • Bond dissociation energy decreases (e.g., F-F bond is weak due to lone pair repulsion).

B. Reaction with Metals (Formation of Salts)

Halogens react with metals to form ionic compounds (salts):

2Na + Cl₂ → 2NaCl (Sodium Chloride)
Ca + Br₂ → CaBr₂ (Calcium Bromide)

• Fluorine is the most reactive → forms ionic bonds even with less reactive metals.
• Iodine is the least reactive → may form covalent bonds (e.g., with hydrogen).

C. Reaction with Hydrogen (Hydrogen Halides)

Halogens react with hydrogen to form hydrogen halides (HX):

H₂ + X₂ → 2HX (where X = F, Cl, Br, I)

• HF is a weak acid (used in toothpaste).
• HCl is a strong acid (used in stomach acid).
• HI is the strongest reducing agent among hydrogen halides.

D. Disproportionation Reactions

Some halogens oxidize and reduce themselves in the same reaction:

Cl₂ + H₂O → HCl + HClO (Chlorine water)

• Chlorine acts as both oxidizing and reducing agent.
• Bromine and iodine also show disproportionation in alkaline medium:

  3Br₂ + 6OH⁻ → 5Br⁻ + BrO₃⁻ + 3H₂O
  

E. Oxidizing Power

• Fluorine is the strongest oxidizing agent (can oxidize even noble gases like Xe).
• Oxidizing power decreases down the group:

  F₂ > Cl₂ > Br₂ > I₂
  

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4. Uses of Halogens

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5. Environmental Impact of Halogens

A. Ozone Depletion by CFCs (Chlorofluorocarbons)

• CFCs (e.g., CFCl₃) were used in refrigerators and aerosols.
• Chlorine atoms from CFCs destroy ozone (O₃) in the stratosphere:

  Cl + O₃ → ClO + O₂
  ClO + O → Cl + O₂
  

• Result: Ozone layer depletion → increased UV radiation → skin cancer, global warming.

B. Toxicity of Halogens

• Chlorine gas (Cl₂) is poisonous (used in WWI as a chemical weapon).
• Iodine vapors are harmful if inhaled.
• Fluorine is extremely corrosive (attacks glass, metals, and even water).

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6. Solved Examples

Example 1: Reaction of Chlorine with Cold & Hot NaOH

Reaction with cold NaOH:

Cl₂ + 2NaOH → NaCl + NaOCl + H₂O

Reaction with hot NaOH:

3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O

Explanation:

• In cold NaOH, chlorine forms sodium hypochlorite (NaOCl) (bleaching agent).
• In hot NaOH, it forms sodium chlorate (NaClO₃) (oxidizing agent).

Example 2: Disproportionation of Bromine in Alkaline Medium

3Br₂ + 6OH⁻ → 5Br⁻ + BrO₃⁻ + 3H₂O

Balancing Steps:

1. Oxidation half: Br₂ → BrO₃⁻ (Br goes from 0 to +5, loses 5e⁻).
2. Reduction half: Br₂ → Br⁻ (Br goes from 0 to -1, gains 1e⁻).
3. Balance electrons: Multiply oxidation by 1, reduction by 5.
4. Add OH⁻ and H₂O to balance O and H.

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7. NEB Board-Style Questions

Short Answer Questions (SAQ)

1. Why does reactivity decrease down Group 17?

   • Answer: Due to increasing atomic size and decreasing bond dissociation energy.

2. Write the reaction of chlorine with cold and hot NaOH.

   • Answer:
     • Cold: Cl₂ + 2NaOH → NaCl + NaOCl + H₂O
     • Hot: 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O

3. What is disproportionation? Give an example.

   • Answer: A reaction where the same element is oxidized and reduced.
   • Example: Cl₂ + H₂O → HCl + HClO

Long Answer Questions (LAQ)

4. Describe the preparation, properties, and uses of chlorine.

   • Preparation:
     • From NaCl (electrolysis):

       2NaCl (l) → 2Na (l) + Cl₂ (g)
       

   • Properties:
     • Greenish-yellow gas, oxidizing agent, reacts with metals/non-metals.
   • Uses:
     • Water purification (bleach), PVC manufacture, disinfectant.

5. Explain the environmental impact of CFCs on the ozone layer.

   • Answer:
     • CFCs release Cl atoms in the stratosphere.
     • Cl acts as a catalyst, breaking O₃ into O₂.
     • Result: Ozone depletion → increased UV radiation.

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Exam Tip

✅ Key Focus Areas for NEB Exam:

1. Trends in Group 17 (reactivity, physical properties, oxidizing power).
2. Disproportionation reactions (chlorine in cold/hot NaOH).
3. Uses of halogens (chlorine in bleach, iodine in antiseptics).
4. Environmental impact (CFCs and ozone depletion).
5. Balancing redox reactions (e.g., bromine in alkaline medium).

❌ Common Mistakes to Avoid:

• Forgetting that fluorine is the most reactive (not chlorine).
• Mixing up cold vs. hot NaOH reactions of chlorine.
• Not balancing disproportionation reactions properly.

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Purple-black solid in a test tube (Image: Dnn87, CC BY 3.0, via Wikimedia Commons) Stratosphere with O₃ breakdown (Image: Mrfoogles, CC BY-SA 4.0, via Wikimedia Commons)