Group 7: the halogens

1. Group 7 contains the halogens: fluorine, chlorine, bromine and iodine.
2. They exist as diatomic molecules, such as ${Cl}_2$ and ${Br}_2$.
3. Each atom has seven outer electrons, so it gains one electron to complete its outer shell, forming a $-1$ halide ion.
4. At room temperature their colour and state change down the group:
   1. Fluorine is a pale yellow gas.
   2. Chlorine is a pale green gas.
   3. Bromine is a red-brown liquid.
   4. Iodine is a grey-black solid.
5. So going down the group the halogens get darker and denser, and their melting and boiling points increase as the molecules get larger.

Reactivity decreases down Group 7

1. Reactivity of the halogens decreases down the group, the opposite of Group 1.
2. Reacting means gaining an electron, so a halogen is more reactive when it can attract an electron more strongly.
3. Down the group, the outer shell is further from the nucleus and there is more shielding from inner shells.
4. This weakens the attraction for an incoming electron, so the electron is gained less easily and the halogen is less reactive.

Displacement reactions show the order of reactivity

1. A more reactive halogen will displace a less reactive halogen from a solution of its salt.
2. For example, chlorine displaces bromine from potassium bromide solution.
   1. $${Cl}_2+2KBr\to 2KCl+{Br}_2$$
   2. The solution changes colour as the displaced bromine is formed.
3. The results follow the reactivity order chlorine > bromine > iodine:
   1. Chlorine displaces bromine and iodine.
   2. Bromine displaces iodine only.
   3. Iodine displaces neither, because it is the least reactive.