The Salt Formers
The halogens — fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At) — constitute Group 17 of the periodic table. Their name comes from the Greek hals (salt) and gennao (to produce): they are literally "the salt producers." React any halogen with an alkali metal, and you get an ionic salt — table salt (NaCl) being the most familiar example.
Halogens are the most reactive group of nonmetals. They are electronegative, oxidizing agents with seven valence electrons — just one short of the stable noble gas configuration. That near-complete outer shell drives their chemistry.
Electron Configuration and Reactivity
Every halogen has the configuration [noble gas] ns²np⁵:
- Fluorine: [He] 2s²2p⁵
- Chlorine: [Ne] 3s²3p⁵
- Bromine: [Ar] 3d¹⁰4s²4p⁵
- Iodine: [Kr] 4d¹⁰5s²5p⁵
- Astatine: [Xe] 4f¹⁴5d¹⁰6s²6p⁵
With seven valence electrons, halogens need only one more electron to complete their octets. They achieve this by: 1. Gaining one electron from a metal → ionic compound (e.g., NaCl) 2. Sharing one electron with a nonmetal → covalent compound (e.g., HCl)
Their dominant oxidation state is –1 (in ionic compounds). Chlorine, bromine, and iodine can also exhibit positive oxidation states (+1, +3, +5, +7) in compounds with oxygen and fluorine, because these heavier halogens have available d orbitals. Fluorine, having no d orbitals available and being the most electronegative element, is always –1.
Physical Properties and States at Room Temperature
Halogens display a vivid progression of physical states and colors at room temperature:
| Halogen | State (25°C) | Color | Boiling Point |
|---|---|---|---|
| Fluorine | Gas | Pale yellow | –188°C |
| Chlorine | Gas | Yellow-green | –34°C |
| Bromine | Liquid | Dark red-brown | 59°C |
| Iodine | Solid | Dark gray/violet | 184°C |
| Astatine | Solid | (predicted black) | ~337°C |
The trend from gas to liquid to solid reflects increasing London dispersion forces (van der Waals forces) as the number of electrons increases and molecules become more polarizable. Larger electron clouds are more easily distorted, leading to stronger intermolecular attractions and higher boiling points.
Reactivity Trend Down the Group
Reactivity decreases down the group — fluorine is the most reactive nonmetal in the entire periodic table, while astatine is far less reactive. This trend has the same origin: decreasing electronegativity and decreasing ability to attract bonding electrons.
Fluorine is so reactive that it attacks glass and reacts with noble gases like xenon and krypton. It was the last of the common halogens to be isolated (1886, by Henri Moissan) precisely because it oxidizes virtually everything, including most containers. It must be handled in special copper or nickel apparatus.
Displacement reactions demonstrate the reactivity order: - Cl₂ + 2KBr → 2KCl + Br₂ (chlorine displaces bromine — chlorine is more reactive) - Br₂ + 2KI → 2KBr + I₂ (bromine displaces iodine) - I₂ does NOT displace bromine or chlorine from their salts
This displacement series directly reflects the reduction potential of the halogens: F₂ > Cl₂ > Br₂ > I₂.
The Halogens as Oxidizing Agents
Halogens are powerful oxidizing agents — they readily accept electrons from other species. Their standard reduction potentials confirm this:
- F₂ + 2e⁻ → 2F⁻ E° = +2.87 V (strongest common oxidizing agent)
- Cl₂ + 2e⁻ → 2Cl⁻ E° = +1.36 V
- Br₂ + 2e⁻ → 2Br⁻ E° = +1.07 V
- I₂ + 2e⁻ → 2I⁻ E° = +0.54 V
Fluorine's extraordinary oxidizing power means it can oxidize water itself: 2F₂ + 2H₂O → 4HF + O₂
Hydrogen Halides
All halogens react with hydrogen to form hydrogen halides:
H₂ + X₂ → 2HX
- HF (hydrogen fluoride): Weak acid in water despite the strong F electronegativity — because the H–F bond is so strong (565 kJ/mol) that it dissociates incompletely. Aqueous HF (hydrofluoric acid) is extremely dangerous — it penetrates skin and attacks calcium in bones and blood.
- HCl (hydrogen chloride): Strong acid in water — essentially 100% dissociated. Major industrial chemical.
- HBr, HI: Both strong acids in water; stronger than HCl.
Note: acid strength of hydrogen halides increases down the group, even though H–X bond polarity decreases. The dominant factor is bond dissociation energy — the H–F bond is too strong, so HF doesn't fully dissociate.
Chlorine: Industrial Giant
Chlorine is produced in massive quantities (~70 million tonnes/year globally) via the chloralkali process: electrolysis of brine (NaCl solution):
2NaCl(aq) + 2H₂O(l) → Cl₂(g) + H₂(g) + 2NaOH(aq)
Chlorine's applications include: - Water disinfection: Cl₂ kills pathogens by oxidizing biological molecules — preventing cholera, typhoid, and other waterborne diseases - PVC (polyvinyl chloride): One of the most common plastics, via vinyl chloride monomer (CH₂=CHCl) - Bleach: Sodium hypochlorite (NaOCl), formed by Cl₂ + NaOH - Pharmaceuticals: ~85% of pharmaceuticals use chlorine chemistry at some stage in synthesis
Iodine in Biology and Medicine
Iodine is essential for human health — it is incorporated into thyroid hormones (thyroxine T₄ and triiodothyronine T₃), which regulate metabolism, growth, and development. Iodine deficiency causes goiter and, in severe cases, cretinism. This is why table salt is iodized in most countries.
The iodine-starch test is a classic chemistry diagnostic: iodine (as I₃⁻) forms a deep blue-black inclusion complex with the helical structure of amylose starch — sensitive enough to detect starch at very low concentrations.
Povidone-iodine (Betadine) is a topical antiseptic using slow-release iodine complexed with polyvinylpyrrolidone.
Astatine: Radioactive and Rare
Astatine is the rarest naturally occurring element — only about 25 grams exist in Earth's crust at any time. All isotopes are radioactive; the most stable is ²¹⁰At (t₁/₂ = 8.1 hours). Despite its rarity, astatine's chemistry is well-studied because it can be produced in particle accelerators.
Most excitingly, ²¹¹At is a candidate for targeted alpha-therapy (TAT) in cancer treatment. Because it decays by alpha emission (highly localized, high energy) and can be attached to tumor-targeting antibodies, it may selectively destroy cancer cells with minimal damage to surrounding tissue.