4.7 Halogens
The elements in Group 17 (or Group VII-A) are called halogens. The name "halogen" is derived from the Greek words "halous" meaning salt and "gen" meaning former. Halogens include fluorine, chlorine, bromine, iodine, astatine, and tennessine (astatine and tennessine are radioactive elements with less known properties). All halogens are reactive non-metals and exist as diatomic molecules.
4.7.1 Appearance of Halogens
Halogens exist as diatomic, colored molecular substances. The color of halogens darkens as you go down the group:
- Fluorine (F₂): Pale yellow gas
- Chlorine (Cl₂): Yellow-green gas
- Bromine (Br₂): Red-brown liquid
- Iodine (I₂): Grey-black solid, turns into dark purple vapors on warming
Electronic Configuration
Halogens possess 7 electrons in their valence shell with the general electronic configuration ns²np⁵. They need only one more electron to complete their valence shell, hence they tend to gain one electron to form univalent negative ions: F⁻, Cl⁻, Br⁻, I⁻.
Density of Halogens
As you move down the group, the number of electrons and protons increases, resulting in a larger atomic size and volume. However, the mass increase is greater than the volume increase, so density generally increases:
| Halogen | Density (g/cm³ at 25 °C) |
|---|---|
| Fluorine | 0.0017 |
| Chlorine | 0.0032 |
| Bromine | 3.1028 |
| Iodine | 4.933 |
Reactivity of Halogens
The reactivity of halogens is related to their ability to gain an electron and form halide ions (F⁻, Cl⁻, Br⁻, I⁻). Fluorine is the most reactive halogen due to its high tendency to gain electrons. Reactivity decreases down the group as electronegativity decreases:
Order of decreasing reactivity: F₂ ≥ Cl₂ ≥ Br₂ ≥ I₂
Displacement Reactions of Halogens
The oxidizing power of F₂ is the highest, and I₂ has the lowest. This means that a free halogen can oxidize or displace the ion of the halogen next to it in the group from their aqueous solutions. For example:
- F₂ can oxidize and displace all other halide ions.
- Cl₂ can oxidize Br⁻ and I⁻ ions.
- I₂ cannot oxidize any halide ion.
Hydrogen Halides and Their Thermal Stabilities
Halogens react with hydrogen to form hydrogen halides:H₂ + X₂ → 2HXwhere X = F₂, Cl₂, Br₂, I₂. The strength of the hydrogen-halogen bond decreases from HF to HI due to the decreasing electronegativity difference between hydrogen and halogen:
- HF ≥ HCl ≥ HBr ≥ HI
Concept Assessment Exercise 4.9
Which of the following displacement reactions will occur?
- Cl₂ + 2NaF → 2NaCl + F₂
- Br₂ + 2KI → 2KBr + I₂
- I₂ + 2KBr → 2KI + Br₂
- Cl₂ + 2KBr → 2KCl + Br₂
- Cl₂ + 2NaI → 2NaCl + I₂