Halogens and Their Compounds
Overview
Halogens are the five elements in Group 17 of the periodic table:
Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At).
These elements exhibit similar chemical behaviors and are collectively known as halogens ("salt-forming elements").
1. Properties of Halogens
1.1 Atomic Properties
- Atomic and ionic radii increase with atomic number.
- Halide ions (X⁻) are larger than their neutral atoms.
- Each halogen has seven valence electrons, and readily forms −1 anions through ionic bonding.
- All elemental halogens exist as diatomic molecules: F₂, Cl₂, Br₂, I₂.
- Except fluorine, halogens exhibit multiple oxidation states (−1 to +7), depending on the compound.
1.2 Physical Characteristics
| Element | State at Room Temperature | Color | Melting Point (°C) | Boiling Point (°C) | Density (g/cm³) | Solubility in Water (g/100g @ 25°C) |
|---|---|---|---|---|---|---|
| Fluorine (F₂) | Gas | Pale yellow | −220 | −188 | 0.00171 | Reacts with water |
| Chlorine (Cl₂) | Gas | Greenish-yellow | −101 | −34 | 0.00321 | Slightly soluble |
| Bromine (Br₂) | Liquid | Reddish-brown | −7 | 59 | 3.14 | 3.48 |
| Iodine (I₂) | Solid | Black-purple | 114 | 184 | 4.93 | 0.034 |
- As atomic number increases, melting and boiling points also increase.
- All halogens are colored and toxic.
1.3 Chemical Behavior
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Halogens are strong oxidizing agents.
Oxidizing strength decreases in the order:
F₂ > Cl₂ > Br₂ > I₂ -
Displacement Reactions:
A stronger halogen can displace a weaker halogen from its salt in aqueous solution:- Cl₂ + 2KBr → 2KCl + Br₂
- Br₂ + 2KI → 2KBr + I₂
-
Reactions with Metals:
Halogens form ionic halides with metals:- 2Na + Cl₂ → 2NaCl
-
Reactions with Nonmetals:
Halogens form covalent halides with nonmetals:- H₂ + Cl₂ → 2HCl (violent reaction under light)
2. Chlorine: Preparation and Properties
2.1 Industrial Method
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Chlorine is produced by electrolysis of concentrated sodium chloride (brine):
2NaCl + 2H₂O → 2NaOH + H₂ + Cl₂↑
2.2 Laboratory Methods
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Reaction of manganese dioxide with concentrated hydrochloric acid:
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂↑ -
Mixture of sodium chloride, sulfuric acid, and manganese dioxide:
2NaCl + 3H₂SO₄ + MnO₂ → MnSO₄ + 2NaHSO₄ + 2H₂O + Cl₂↑- Gases passed through water to remove HCl, and through concentrated H₂SO₄ for drying.
-
Reaction of bleaching powder [CaCl(OCl)·H₂O] with hydrochloric acid:
- Also generates Cl₂ gas for small-scale use.
3. Summary
- Halogens are reactive, colored, toxic elements that readily form halides.
- Fluorine is the strongest oxidizer, iodine the weakest.
- Elemental halogens increase in melting/boiling points down the group.
- Chlorine can be prepared both industrially and in laboratories using redox reactions.
- Halogen chemistry is foundational in salt production, organic synthesis, disinfection, and more.