2bGroup 7 (halogens) – chlorine, bromine and iodine

The halogens are the clearest example in the whole Periodic Table of a physical trend you can see with your eyes.

Colour and state

Two trends running together: going down the group the elements get darker, and they change from gas to liquid to solid.

Questions almost always want both the colour and the state. Giving one earns half the answer.

Why the state changes

All the halogens are simple molecular, existing as diatomic molecules — Cl₂, Br₂, I₂.

Going down the group the molecules get larger, so the intermolecular forces get stronger, so more energy is needed to separate them, so the melting and boiling points rise.

That is why chlorine is already a gas at room temperature while iodine is still a solid.

Notice what is not changing: the covalent bond inside each molecule. As always with molecular substances, melting and boiling concern only the forces between molecules.

Two trends, opposite directions

This is the point students most often tangle:

• Boiling point increases down Group 7.
• Reactivity decreases down Group 7.

They run opposite ways because they depend on different things — boiling point on the size of the molecules, reactivity on how strongly an atom attracts an incoming electron.

So iodine is the highest boiling and the least reactive of the three. Both are correct at once.

Reactivity decreases down Group 7

Fluorine is the most reactive halogen, at the top. Iodine is the least reactive of the three you study. Astatine, below iodine, would be less reactive still.

Going up the group means more reactive — the opposite of Group 1, and worth saying to yourself once more.

Displacement

A more reactive halogen displaces a less reactive one from its compound.

Add chlorine solution to colourless potassium bromide solution and it turns orange. Chlorine has displaced the bromine, and the orange colour is the bromine that has been set free.

Cl₂ + 2KI → 2KCl + I₂

Both halogens are diatomic, which is why the potassium compounds need a coefficient of 2.

Reading a results table

From these three rows the order follows: chlorine, then bromine, then iodine.

The "no change" row is evidence too, and answers that ignore it usually fall short of full marks. Bromine failing to displace chlorine confirms that chlorine is the more reactive — it rules out the alternative ordering.

Predicting whether a reaction happens

Compare the halogen you are adding with the one already in the compound:

• Added halogen higher in the group → it displaces, and you see a colour change.
• Added halogen lower → nothing happens.

So chlorine displaces both bromine and iodine; iodine displaces neither.

The colour is the evidence

When describing what you would see, state the starting colour as well as the final one. "It turns brown" means much less than "the colourless solution turns brown" — the change is the observation, not the end state alone.

Separate Chemistry only.

Halogens react by gaining one electron

Every Group 7 atom has seven outer-shell electrons and needs one more to complete the shell. Reacting means capturing that electron and becoming a 1− ion.

That single fact is the whole explanation, because it makes the trend the mirror image of Group 1's.

Why it gets harder down the group

Going down Group 7:

1. The outer shell is further from the nucleus, because there are more shells.
2. It is better shielded by the inner shells.

So an incoming electron is attracted less strongly, is gained less easily, and the element is less reactive.

Fluorine, and why it is so violent

Fluorine sits at the top with the fewest shells. Its outer shell is closest to the nucleus with the least shielding, so it attracts an incoming electron more strongly than any other halogen.

That is why fluorine's displacement reactions cannot safely be done in a school laboratory.

The two groups, one cause

This is worth holding as a single idea rather than two separate facts:

The middle row is identical. The trends diverge only because losing and gaining are opposite operations.

If you can reconstruct that table, you never have to memorise which group goes which way.