Introduction to Halogens
What are Halogens?
Halogens are elements present in Group 17 of the periodic table. The name “halogen” comes from the Greek words “halos” (salt) and “gen” (to make).
- Fluorine (F) – Pale yellow gas
- Chlorine (Cl) – Greenish yellow gas
- Bromine (Br) – Reddish-brown liquid
- Iodine (I) – Shiny greyish-black solid
- Astatine (At) – Radioactive element
- Tennessine (Ts) – Radioactive element
Key Point: Halogens exist as diatomic molecules (F₂, Cl₂, Br₂, I₂) in all phases and are very reactive non-metals.
Physical and Chemical Properties
Colors and Physical States
- Fluorine: Pale yellow gas at room temperature
- Chlorine: Greenish yellow gas at room temperature
- Bromine: Reddish-brown volatile liquid
- Iodine: Shiny greyish-black solid that sublimes to violet vapor
Atomic Properties
| Element | Proton No. | State at 20°C | Melting Point (°C) | Boiling Point (°C) |
|---|---|---|---|---|
| Fluorine | 9 | Gas | -220 | -188 |
| Chlorine | 17 | Gas | -101 | -35 |
| Bromine | 35 | Liquid | -7 | 59 |
| Iodine | 53 | Solid | 113 | 183 |
Trend in Volatility
Volatility Trend Down Group 17
Volatility decreases from chlorine to iodine due to:
- Increasing molecular mass
- Increase in size of outer shell
- Stronger intermolecular forces (London dispersion forces)
Why does volatility decrease down Group 17?
Volatility decreases down the group because London dispersion forces increase with increasing molar mass and size. Larger, more polarizable molecules have stronger intermolecular forces, requiring more energy to vaporize.
Reactivity Trends
Oxidizing Power
The oxidizing power of halogens decreases down the group:
F₂ > Cl₂ > Br₂ > I₂
- Fluorine is the strongest oxidizing agent
- Iodine is the weakest oxidizing agent
- Chlorine can displace Br⁻ and I⁻ from solutions
- Bromine can displace I⁻ from solutions
- Iodine cannot displace any halide ions
Reactions with Hydrogen
- F₂ + H₂: Explosive even at low temperature in dark
- Cl₂ + H₂: Reacts in UV light or with spark
- Br₂ + H₂: Reacts upon heating
- I₂ + H₂: Slow, reversible reaction at high temperature with catalyst
Applications of Halogens
Water Purification with Chlorine
Chlorine is widely used in water treatment due to its disinfectant properties:
- Added to water, it forms HCl and HOCl
- HOCl dissociates to H⁺ and OCl⁻
- Both HOCl and OCl⁻ are effective disinfectants
- HOCl is more effective due to neutral charge
Chemical Equation: Cl₂ + H₂O → HCl + HOCl
Other Applications
- Fluorine: Used in toothpaste (fluoride), Teflon production
- Bromine: Fire retardants, photography, pharmaceuticals
- Iodine: Antiseptics, thyroid hormones, photography