Seven outer electrons, one gap
One electron short of a noble-gas configuration, so they form −1 ions with great enthusiasm. "Halogen" means salt-former: combine one with a metal and you get a salt, of which table salt is only the most famous.
Group 17 — the salt formers, one electron short of contentment
Halogens
Fluorine, chlorine, bromine, iodine and astatine need exactly one more electron to complete their outer shell, and they will take it from almost anything. That single-minded appetite makes them the most reactive nonmetals in the table — and the reason your water is safe to drink.
- Members
- 5
- Group
- 17
- Configuration
- ns² np⁵
- Radioactive
- 1
Highlighted in the periodic table
Every member
The 5 halogens
9
F
Fluorine
18.998 u
The most electronegative element known: fluorine takes electrons from almost anything, including glass and water.
17
Cl
Chlorine
35.45 u
A pale green gas that kills bacteria, bleaches fabric — and, bound to sodium, sits on your dinner table.
35
Br
Bromine
79.904 u
One of only two elements that are liquid at room temperature, and the one that smells like a swimming pool from hell.
53
I
Iodine
126.904 u
The halogen your thyroid cannot function without — and the reason table salt is iodised.
117
Ts
Tennessine
294 u
The second-to-last gap in the seventh period, filled in 2010 with a berkelium target.
What defines them
Three things that make a halogen
Reactivity decreases downward
Fluorine is the most electronegative element there is and reacts with nearly everything, including glass and water. Chlorine is aggressive, bromine less so, iodine mild enough to be sold in a pharmacy. Each step down puts the incoming electron further from the nucleus.
Diatomic, and a full set of phases
All exist as X₂ molecules, and the group runs through every phase at room temperature: fluorine and chlorine are gases, bromine is one of only two liquid elements, iodine and astatine are solids.
Periodic trends
How they change across the family
Down the group: atomic radius grows, electronegativity and reactivity fall, and melting and boiling points rise as heavier molecules attract each other more strongly. Halogens displace each other from solution in strict order — chlorine will pull bromide out of a bromide salt, but bromine cannot touch a chloride.
In the world
What they are used for
Fluorine
PTFE (Teflon), refrigerants, uranium enrichment, and fluoride in toothpaste and drinking water — a measure that cut tooth decay dramatically.
Chlorine
Water disinfection, PVC, bleach and a vast fraction of all industrial chemistry. Chlorination of drinking water is arguably the single most life-saving public-health intervention ever deployed.
Bromine
Flame retardants, agricultural chemicals and photographic emulsions, though many uses have been restricted.
Iodine
Antiseptics, contrast media and — critically — dietary supplementation. Iodised salt eliminated goitre and iodine-deficiency brain damage across most of the world.
Astatine
Research only. It is the rarest naturally occurring element on Earth.
Handling
Safety
As a class, the most immediately dangerous nonmetals. Fluorine gas reacts violently with almost anything, and hydrofluoric acid penetrates skin painlessly to attack bone. Chlorine gas was the first chemical weapon used at scale in 1915 and remains a serious industrial hazard. Bromine causes deep chemical burns. Iodine, by contrast, is safe enough at low concentration to swab a wound with.
Worth knowing
- Isolating fluorine took chemists 74 years and blinded or killed several of them; Henri Moissan finally succeeded in 1886 and won a Nobel for it.
- Bromine and mercury are the only two elements that are liquid at room temperature and pressure.
- Iodine sublimes directly from a purple-black solid to a violet vapour, which is where its name comes from.
Common questions
About the halogens
Why is fluorine the most reactive element?
Two reasons compound. Its small atom means an incoming electron ends up very close to the nucleus, which is energetically excellent. And the F–F bond in fluorine gas is unusually weak — the two lone-pair-heavy atoms repel each other — so very little energy is needed to break the molecule apart before it attacks something.
Is fluoride in water safe?
At the levels used for water fluoridation, roughly 0.7 mg/L, the evidence for safety is extensive and the reduction in tooth decay is well documented. Fluoride is toxic at much higher doses, and naturally fluoride-rich groundwater in parts of India and Africa does cause skeletal fluorosis. Dose is the whole story.
Why is astatine so rare?
It has no stable isotope, and its longest-lived one has a half-life of 8.1 hours. The only astatine on Earth is being produced continuously in trace amounts as an intermediate in uranium and thorium decay chains, and it decays as fast as it forms. Best estimates put the total present in the entire crust at under 30 grams at any moment.
Keep going
The other nine families
6
Alkali metals
Li Na K Rb Cs Fr
6
Alkaline earth metals
Be Mg Ca Sr Ba Ra
38
Transition metals
Sc Ti V Cr Mn Fe Co
12
Post-transition metals
Al Ga In Sn Tl Pb Bi
7
Metalloids
B Si Ge As Sb Te At
7
Nonmetals
H C N O P S Se
7
Noble gases
He Ne Ar Kr Xe Rn Og
15
Lanthanides
La Ce Pr Nd Pm Sm Eu
15
Actinides
Ac Th Pa U Np Pu Am