Halogens

Group 17 is the most predictable column in the periodic table and, at the same time, the one that contains the single most aggressive element there is. Fluorine, chlorine, bromine, iodine and astatine each need one more electron to complete an outer shell, and every one of them will go a long way to get it. The word halogen means "salt-former", which is what happens the moment one of them meets a metal: sodium and chlorine give table salt, potassium and iodine give the additive that eliminated goitre from most of the world.

What makes the group worth studying as a group rather than as five separate elements is that almost every property changes smoothly as you go down it, and the few that do not are the interesting ones.

What they have in common

Each halogen has seven electrons in its outermost shell: an s² p⁵ configuration. That single gap drives everything.

• Of the elements in their period that readily form bonds, they are the most electronegative. Fluorine sits at the top of the Pauling scale at 3.98, and the gap back to oxygen is more than half a unit — wider than the gap between oxygen and nitrogen, and wider than any other single step near the top of the scale.
• They exist as diatomic molecules in their elemental form — F₂, Cl₂, Br₂, I₂ — because two atoms sharing a pair of electrons is the cheapest way for each to fill its shell.
• Their characteristic oxidation state is −1, achieved by taking one electron. The heavier members can also be pushed into positive states by oxygen or fluorine; the lightest cannot.
• They form hydrogen halides — HF, HCl, HBr, HI — which dissolve in water to give acids.

That first point carries a hedge, and the hedge is doing work. Krypton and xenon have Pauling values on this site, worked back from their fluorides, and krypton lands at 3.00 — above bromine's 2.96. In period 4, then, the highest number on the table does not belong to the halogen. Period 6 fails the claim from the other direction: astatine's 2.2 is an extrapolation rather than anything derived from a bond energy, and gold's 2.54 stands above it. The tidy textbook form of the rule needs the noble-gas column left blank and the bottom of group 17 taken on trust, and neither of those is done here.

The one thing they do not share is a place in living chemistry. Chlorine and iodine are essential to humans in quite different ways; fluorine is not essential at all, though it is useful to enamel; bromine's role in animals was only recently established; astatine has no biological presence because there is barely any of it anywhere.

The trend down the group, and why it happens

Every property that depends on how tightly the nucleus holds the outermost shell weakens as you descend, because each step down adds a full shell of electrons between the nucleus and the outside world.

Electronegativity falls from 3.98 in fluorine to 3.16 in chlorine, 2.96 in bromine and 2.66 in iodine. The nucleus is further away and better shielded, so its pull on an incoming electron is weaker.

Reactivity falls with it. Fluorine reacts with almost every element including some noble gases; chlorine needs more encouragement; iodine is sluggish enough to be handled as a solid in a school laboratory. This is exactly the reverse of the alkali metals in group 1, where reactivity rises down the column — and the reason is the same in both cases. Group 1 wants to lose an electron and finds it easier the further out that electron sits; group 17 wants to gain one and finds it harder.

Melting and boiling points rise. Fluorine boils at 85 K and chlorine at 239 K, both gases at room temperature. Bromine boils at 332 K, which makes it a liquid — one of only two elements that are liquid at 20 °C, the other being mercury. Iodine is a solid that sublimes to a violet vapour. The molecules get larger and more polarisable as you descend, so the London dispersion forces holding them together get stronger.

Colour deepens in step with that: pale yellow fluorine, yellow-green chlorine, red-brown bromine, near-black iodine with a purple vapour. The absorption edge moves through the visible spectrum as the molecules get bigger.

Displacement, and how to read it

Put chlorine into a solution of potassium bromide and the solution turns orange: the chlorine takes the electrons and the bromine is displaced as Br₂. Put bromine into potassium chloride and nothing happens at all. A more reactive halogen displaces a less reactive one from its compounds, never the other way round.

That single experiment orders the group by reactivity without any theory at all, which is why it is a fixture of school chemistry. It also gives an honest ranking that matches the electronegativity numbers, which is a satisfying thing to be able to demonstrate rather than assert.

Fluorine is not just "more of the same"

The lightest member of a group is usually the odd one out, and fluorine is the clearest example in the whole table.

It has no positive oxidation states, because nothing is electronegative enough to take electrons from it. Hydrofluoric acid is a weak acid in water while HCl, HBr and HI are all strong — the H–F bond is so strong that it does not fully dissociate, which is the opposite of what the trend predicts. And elemental fluorine resisted isolation for most of a century: several chemists were injured and at least two died attempting it before Henri Moissan succeeded in 1886 by electrolysing potassium bifluoride in anhydrous hydrogen fluoride at −50 °C.

Fluorine also holds the group's practical high ground. Polytetrafluoroethylene, uranium enrichment via UF₆, most modern refrigerants, and a large share of pharmaceuticals all depend on carbon–fluorine bonds, which are the strongest single bonds carbon commonly forms.

The fifth member is a trend nobody can check

Everything above is a line drawn through four points and extended to a fifth. Astatine exists in the crust only as a fleeting branch of the uranium and thorium decay chains, and the published estimates of the whole planet's standing inventory disagree by more than an order of magnitude, which is itself the clearest available statement of how little is known. Nobody has ever had a visible amount of it. Most of what a table prints for astatine is the extension of the line, not a point on it.

That matters more than it sounds, because the line may stop working at the bottom. Astatine's behaviour in solution is not simply a weaker version of iodine's; it shows a positive-ion chemistry that has no counterpart higher in the column, and calculations on the condensed element make it a metal rather than a molecular solid. A group defined by an appetite for one extra electron may have a member at the bottom that does not share it. What has actually been measured, and what remains extrapolation, is set out on the astatine page.

Tennessine, element 117, sits below astatine and is sometimes drawn in the halogen column for that reason. This site does not colour it as a halogen, because its chemistry has never been observed and calling it one would be a prediction dressed up as a fact.

Where each one turns up

• Fluorine — fluorite ore, toothpaste and water fluoridation, PTFE, refrigerants, and around a fifth of all marketed pharmaceuticals.
• Chlorine — seawater and rock salt, PVC, water treatment, bleach, and the chlor-alkali process that also supplies most industrial sodium hydroxide.
• Bromine — brines in Arkansas, Israel and China; flame retardants, drilling fluids, and historically photographic silver bromide.
• Iodine — Chilean caliche and Japanese gas-field brines; iodised salt, X-ray contrast media, and antiseptics.
• Astatine — trace decay chains only, and a small research interest in astatine-211 for targeted alpha therapy.

Getting them out of the ground is mostly electricity

None of the halogens occurs free in nature, for the obvious reason that anything that reactive has already reacted. Every one of them has to be prised out of a halide, and because a halide ion is already at its most comfortable, that means forcing electrons off it.

Chlorine comes from the chlor-alkali process: brine is electrolysed, chlorine comes off the anode, hydrogen off the cathode, and sodium hydroxide is left in solution. The economics of that single process tie the price of three industrial commodities together, which is why a glut of caustic soda can depress the price of chlorine and vice versa.

Fluorine needs the same idea taken to an extreme, because there is no chemical oxidant strong enough to take an electron from a fluoride ion. Moissan's electrolytic route is still essentially the industrial one, and elemental fluorine is so awkward to store and ship that most plants that need it make it on site.

Bromine and iodine are gentler. Both are displaced from their brines by chlorine — precisely the displacement reaction that ranks the group — then stripped out with steam or air and condensed. Iodine is often recovered instead from iodate in Chilean nitrate deposits, by reduction with sulfur dioxide, which avoids handling chlorine on a site that has none.

Where group 17 usually goes wrong in an exam

Four mistakes account for most of the marks lost on this group, and all four come from applying a group-1 habit of mind to group 17.

1. Reversing the reactivity trend. Reactivity increases down group 1 and decreases down group 17. Both are consequences of the same shielding argument, applied to losing an electron in one case and gaining one in the other.
2. Calling HF a strong acid. It is the only weak one of the four hydrogen halides, and it is weak because the H–F bond is unusually strong, not because fluorine is unusually shy.
3. Assuming bromine is a gas. It is one of two elements liquid at room temperature. Marking it as a gas because chlorine and fluorine are is a common slip.
4. Giving fluorine a positive oxidation state. It never has one. Nothing in the periodic table can take an electron from it.

What a halogen is not

The group's boundary is a matter of position, not of behaviour, and it is worth being clear about where the label stops. Hydrogen has one electron short of a full first shell and forms hydrides with metals, which looks halogen-like on paper, but it sits in group 1 and behaves nothing like fluorine. Meanwhile the halogens themselves are coloured on this site as reactive nonmetals rather than given a colour of their own, because "halogen" describes a column and the colour system describes chemical character. Both facts are true at once, and each page says which one it is using.