The halogens
Group 17 contains the halogens: fluorine (F), chlorine (Cl), bromine (Br) and iodine (I). Each atom has seven valence electrons and gains one electron to form a −1 ion with a stable octet. They exist as diatomic molecules: F2, Cl2, Br2, I2.
Physical trends
Going down the group, colour deepens and physical state changes as molecules get larger.
| Halogen | Colour / State |
|---|---|
| Chlorine | Greenish-yellow gas |
| Bromine | Reddish-brown liquid |
| Iodine | Purple-black solid |
Melting and boiling points increase down the group because larger molecules have stronger forces of attraction between them.
Trend in reactivity
Reactivity decreases down the group. As atoms get larger, the incoming electron is attracted less strongly by the nucleus, so an electron is gained less easily.
Worked example
A more reactive halogen displaces a less reactive one from its salt.
Cl2(aq) + 2KBr(aq) → 2KCl(aq) + Br2(aq)
Chlorine (higher in the group, more reactive) displaces bromine, turning the solution orange-brown.
Remember
- Seven valence electrons → −1 ions.
- Exist as diatomic molecules.
- Reactivity decreases down the group.
- A more reactive halogen displaces a less reactive one.