What Are the Halogens?
The halogens are the elements in Group 7 of the periodic table: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).
"Halogen" means "salt-former" in Greek — they react with metals to form ionic salts (e.g. sodium chloride, NaCl — table salt).
All halogens consist of diatomic molecules (pairs of atoms): F₂, Cl₂, Br₂, I₂.
Physical Properties
| Element | State at room temp | Colour | Notes |
|---|---|---|---|
| Fluorine (F₂) | Gas | Pale yellow | Extremely reactive |
| Chlorine (Cl₂) | Gas | Yellow-green | Toxic, distinctive smell |
| Bromine (Br₂) | Liquid | Red-brown | Dangerous vapour |
| Iodine (I₂) | Solid | Dark grey/purple | Sublimes on warming |
Why Are They Reactive?
Each halogen has seven electrons in its outer shell and needs just one more to complete it. This strong tendency to gain an electron makes halogens highly reactive (non-metals that gain electrons form negative ions: Cl⁻, F⁻, Br⁻, I⁻).
Reactivity decreases going down the group. Fluorine is the most reactive element known — it reacts explosively with almost everything. Iodine is much less reactive. This is because lower halogens have more electron shells, so the attraction for an extra electron is weaker.
Reactions with Metals
Halogens react with metals to form metal halides (salts):
Displacement Reactions Between Halogens
A more reactive halogen can displace a less reactive one from a solution of its salt:
Chlorine displaces bromine (orange-brown colouration in the solution).
Uses of Halogens
- Chlorine: water purification, swimming pools, bleach, PVC plastic
- Fluorine: in toothpaste (fluoride strengthens enamel); non-stick coatings (PTFE/Teflon)
- Iodine: antiseptic (tincture of iodine); thyroid function in the body
Real world: Chlorine in swimming pool water
Chlorine (Cl₂) is added to swimming pools in safe concentrations to kill bacteria, viruses, and algae. It reacts with water to form hypochlorous acid (HOCl), which oxidises microorganism cell membranes. This is a direct application of chlorine's high reactivity — its need to gain an electron makes it an aggressive oxidiser.
Real world: Fluoride in toothpaste
Toothpaste contains sodium fluoride (NaF) or sodium monofluorophosphate. Fluoride ions (F⁻) replace hydroxyl groups in tooth enamel, making it more resistant to acid attack from bacteria. Fluorine is the most reactive halogen — at the top of Group 7, it has the strongest attraction for electrons — and its compounds are extremely stable.
Real world: Iodine as a wound antiseptic
Iodine solution (tincture of iodine) is used to disinfect wounds. Iodine molecules (I₂) oxidise proteins in bacteria, killing them. Although less reactive than chlorine or bromine (it is lower in Group 7), iodine is safe enough to apply to skin in dilute solution. It is also essential in your diet — iodine deficiency causes goitre, a swelling of the thyroid gland.
Where do you see this in real life? Swimming pool "chlorine" tabs, fluoride toothpaste, iodine antiseptic liquid, non-stick Teflon (PTFE, a fluorine compound) on frying pans — halogens are part of daily life, usually in safer compound or diluted forms.