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The halogens are the elements of group 17: fluorine, chlorine, bromine, iodine, astatine and the superheavy tennessine. Their name means “salt-formers” (from Greek hals, salt), because they react with metals to make salts — sodium chloride being the most famous.
They’re the most reactive nonmetals, and they’re the only group on the periodic table that contains elements in all three everyday states at room temperature.
Seven outer electrons
Each halogen has seven electrons in its outer shell (ns² np⁵) — one short of a full, noble-gas configuration. Gaining one electron completes the shell, so halogens readily form −1 ions (F⁻, Cl⁻, Br⁻, I⁻), called halides.
As elements, they exist as diatomic molecules: F₂, Cl₂, Br₂, I₂. Two atoms share a pair of electrons so that each has a full octet. See diatomic elements.
A group of three states and many colours
| Element | State at 25 °C | Colour | Melting point (°C) | Boiling point (°C) |
|---|---|---|---|---|
| Fluorine | Gas | Pale yellow | −219.6 | −188.1 |
| Chlorine | Gas | Yellow-green | −101.5 | −34.0 |
| Bromine | Liquid | Dark red-brown | −7.2 | 58.8 |
| Iodine | Solid | Shiny grey-black; purple vapour | 113.7 | 184.4 |
| Astatine | Solid (predicted) | Unknown — never seen | ~302 (est.) | — |
The colours deepen and the melting and boiling points rise down the group. Bigger molecules have more electrons, so the London dispersion forces between them get stronger — enough to turn a gas into a liquid and then a solid.
Reactivity decreases down the group
This is the opposite of the alkali metals. Halogens react by gaining an electron. Going down the group:
- atoms get bigger,
- the incoming electron is further from the nucleus and more shielded,
- so it’s attracted less strongly.
Fluorine is the most reactive of all — and the most electronegative element. It reacts with almost everything, including glass, water and even some noble gases. Iodine is much milder.
Displacement reactions
A more reactive halogen displaces a less reactive one from its salts:
Cl₂ + 2KBr → 2KCl + Br₂ (the solution turns orange) Cl₂ + 2KI → 2KCl + I₂ (turns brown) Br₂ + 2KI → 2KBr + I₂
But I₂ + KCl → no reaction, because iodine is less reactive than chlorine. These reactions are a classic way to demonstrate the order of reactivity. They’re also how bromine is extracted industrially: chlorine gas is bubbled through bromide-rich brine.
Hydrogen halides and acids
Halogens combine with hydrogen to form hydrogen halides (HF, HCl, HBr, HI), which dissolve in water to give acids. HCl, HBr and HI are strong acids — but HF is weak, because the H–F bond is so strong.
Uses
- Fluorine: fluoride in toothpaste and water to prevent tooth decay; PTFE non-stick coatings; refrigerants; uranium hexafluoride for nuclear fuel enrichment; many modern medicines contain fluorine atoms.
- Chlorine: disinfecting drinking water and swimming pools (one of the most important public-health advances in history), bleach (sodium hypochlorite), PVC plastic, and countless chemicals.
- Bromine: flame retardants, some medicines, photographic film (silver bromide), and water treatment in hot tubs.
- Iodine: antiseptic (iodine tincture, povidone-iodine), iodized salt to prevent thyroid problems, and X-ray contrast agents.
- Astatine: astatine-211 is being studied for targeted cancer therapy. See the rarest element on Earth.
Safety
Elemental fluorine, chlorine and bromine are toxic and corrosive. Chlorine was used as a chemical weapon in the First World War. The halides — the ions — are a completely different story: chloride is essential to life, and we eat it every day in salt. That contrast between an element and its ions is a good reminder of how much chemistry changes when electrons move.
Tennessine
Element 117 sits at the bottom of the group, but it’s known only from a few dozen atoms, each lasting fractions of a second. Calculations suggest it may behave more like a metalloid than a typical halogen. See the newest elements.
Quick answers
Which halogen is the most reactive? Fluorine.
Which halogen is a liquid at room temperature? Bromine — one of only two liquid elements. See liquid elements.
Why are halogens diatomic? Sharing one pair of electrons gives each atom a full outer shell.
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