The periodic table · Contrast
Group 7 — the halogens
Group 1 got fiercer the further down you went. Group 7 is the same kind of column with the same kind of family — so does it do the same thing?
Start here
Three sealed tubes on the bench: a pale green gas, a red-brown liquid and a grey-black solid.
They are all in the same group of the periodic table, one under the other. Nothing about them looks like a family: a gas, a liquid and a solid, in three different colours. Warm the grey-black solid gently and it turns straight into a violet vapour.
If they are so different, why are they in the same group?
Because a group is about chemical behaviour, not appearance. All three react with metals to make the same kind of compound — a chloride, a bromide, an iodide, all with the same formula pattern. Whether an element happens to be a gas, a liquid or a solid at room temperature is a detail about melting points, and those change steadily down the column. The chemistry is the family resemblance.
Group 7 — the halogens — are coloured, poisonous non-metals. Going down the column they get darker and their melting points rise, which is why the top of the group is a gas and the bottom is a solid.
And going down, they get less reactive. Fluorine at the top is the most reactive element in the whole table; iodine near the bottom is mild enough to be painted on skin as an antiseptic. That is the reverse of group 1, and it is the fact this lesson is built on.
Reference · keep this one open
The four you need
- FluorinePale yellow gasThe most reactive element in the table
- ChlorineGreen gas, chokingVery reactive — kills bacteria in tap water
- BromineRed-brown liquid, orange vapourReactive, but less so than chlorine
- IodineGrey-black solid, violet vapourThe mildest of the four — used as an antiseptic
Read the last column downwards. Group 1 got fiercer as you descended; this one calms down.
Your turn · nine tubes
Three halogens, three salt solutions. Pick a cell, predict, then run it.
| Added to | potassium chloride | potassium bromide | potassium iodide |
|---|---|---|---|
| Chlorine water | |||
| Bromine water | |||
| Iodine water |
Predict before you run it.
Pick a cell to set up the tube.
Chlorine water added to potassium chloride
A few cm³ of colourless potassium chloride solution, with pale green chlorine water added on top and shaken.
No reaction. Nothing changes.
The pale green stays exactly as pale green as it went in. Chlorine cannot displace itself, and there is nothing else here for it to take.
Chlorine water added to potassium bromide
A few cm³ of colourless potassium bromide solution, with pale green chlorine water added on top and shaken.
A reaction. The colour changes.
The colourless solution turns orange. That orange is bromine, pushed out of its own salt by the more reactive chlorine.
chlorine + potassium bromide potassium chloride + bromine
Chlorine water added to potassium iodide
A few cm³ of colourless potassium iodide solution, with pale green chlorine water added on top and shaken.
A reaction. The colour changes.
The colourless solution turns brown almost at once. The brown is iodine, displaced by chlorine from two places above it.
chlorine + potassium iodide potassium chloride + iodine
Bromine water added to potassium chloride
A few cm³ of colourless potassium chloride solution, with orange bromine water added on top and shaken.
No reaction. Nothing changes.
Nothing. The orange of the bromine water is the only colour in the tube and it does not change. Bromine is below chlorine, so it cannot take chlorine's place.
Bromine water added to potassium bromide
A few cm³ of colourless potassium bromide solution, with orange bromine water added on top and shaken.
No reaction. Nothing changes.
The orange stays orange. An element cannot displace itself — there is no more reactive halogen in this tube than the one already in the salt.
Bromine water added to potassium iodide
A few cm³ of colourless potassium iodide solution, with orange bromine water added on top and shaken.
A reaction. The colour changes.
The orange darkens to brown. Bromine could not beat chlorine, but it beats iodine — which places it between the two.
bromine + potassium iodide potassium bromide + iodine
Iodine solution added to potassium chloride
A few cm³ of colourless potassium chloride solution, with brown iodine solution added on top and shaken.
No reaction. Nothing changes.
No change at all. Iodine is the least reactive of the three and chlorine is the most, so this is the furthest a displacement could be from happening.
Iodine solution added to potassium bromide
A few cm³ of colourless potassium bromide solution, with brown iodine solution added on top and shaken.
No reaction. Nothing changes.
No change. Bromine is above iodine in the group, so iodine cannot take its place — even though it managed nothing anywhere else either.
Iodine solution added to potassium iodide
A few cm³ of colourless potassium iodide solution, with brown iodine solution added on top and shaken.
No reaction. Nothing changes.
The brown stays brown, which is the third time an element has met its own salt and done nothing. That result is as real as the three that changed colour.
Half the grid is empty, and it is the other half from last time.
Chlorine displaced both the others. Bromine displaced only iodine. Iodine displaced nothing at all. Read that as an order and it says chlorine, then bromine, then iodine — which is the order they appear going down the column.
Reactivity decreases going down group 7.
Same experiment you ran on the metals, same logic, opposite direction. A group has a trend; it is not always the same trend.
Key fact
The halogens get darker, denser and less reactive going down the group. A more reactive halogen displaces a less reactive one from its salt — never the other way round.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Reactivity always increases going down a group.”
Group 1 was unambiguous about it. Commit before you read on.
The two groups do opposite things because they want opposite things. A group 1 atom reacts by losing its outer electron, and the further out that electron sits, the easier it goes — so bigger atoms lower down react more. A group 7 atom reacts by gaining an electron, and a bigger atom pulls an incoming electron in less strongly — so bigger atoms lower down react less.
One rule about atom size, two opposite trends. Learn the reason and you never have to remember which group goes which way.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What happens to reactivity going down group 7?
Rung 2 · The one that catches people
Bromine water is added to potassium chloride solution and nothing happens. Why not?
Rung 3 · Explain
Chlorine water is added to colourless potassium iodide solution. Describe what you would see, name the products, and explain why the reaction happens.
Rung 4 · Take it somewhere new
Astatine sits below iodine in group 7. Predict its appearance and its reactivity, and explain how you would test your prediction about reactivity using only the halogens in this lesson.
Key note
The halogens are coloured, poisonous non-metals that exist as pairs of atoms. Going down the group they become darker and change from gas to liquid to solid, and they become less reactive. A more reactive halogen displaces a less reactive one from a solution of its salt, which is how their order is established.
Going further
Chlorine is the reason tap water is safe. Added at about one part per million it kills the bacteria that cause cholera and typhoid, and the introduction of chlorination to city water supplies saved more lives than almost any medicine. The same element was released as a weapon in 1915. A substance is not good or evil — what is done with it is.
Fluorine is so reactive that isolating it killed or injured several chemists before Henri Moissan managed it in 1886, and it still has to be handled in vessels that have been deliberately coated with their own fluoride. Yet its compounds are the tamest things imaginable: fluoride in toothpaste, and PTFE — a chain of carbon wrapped in fluorine — as the coating on a non-stick pan. An element and its compounds are different substances, and this group proves it four times over.
Before this lesson
Next in this unit
At GCSE this becomes
- Explaining both trends with atomic radius and shielding, displacement as electron transfer, and writing ionic half-equations for the halogens.
Where to next
Ask Mr Badmus AI
Still not sure why this group runs the other way?
Chlorine, bromine and iodine are all toxic and their vapours are harmful. The nine tubes on this page are a simulation of a demonstration done in a fume cupboard, and the halogen solutions are handled by the teacher. Bromine water in particular burns skin.
Lesson content © MrBadmusAI.