Group 7 (halogens)

• Chlorine is a yellow-green gas.
• Bromine is a red-brown liquid.
• Iodine is a purple solid.
• There is a trend in state from gas to liquid to solid down the group.
• This is because the melting and boiling points increase as you go down the group.
• From this, you can predict that any halogens above chlorine will be gases (their boiling points will be even lower), and any below iodine will be solids (their melting points will be even greater).
• Displacement reactions involving halogens and halides provide evidence for the trend in reactivity in Group 7.
• A more reactive halogen can displace a less reactive in an aqueous solution of its salt.
• Chlorine will displace bromine if you bubble the gas through a solution of potassium bromide: Chlorine + Potassium Bromide → Potassium Chloride + Bromine.
• Chlorine will displace bromine and iodine.
• Iodine can replace neither chlorine or iodine.
• Bromine will displace iodine but not chlorine.
• The trend in reactivity in Group 7 can be explained in terms of electronic configurations.
• The halogens react by gaining an electron in their outer shell, as you go down the group: outer shell becomes further from the nucleus, electron shielding increases, attraction decreases between nucleus and outer electrons, electrons are gained less easily, and halogens become less reactive.