Group 7, The Halogens
- The halogens exist as diatomic (X2) molecules
- Down the group electronegativity decreases, as atomic radius and shielding increases
- Down the group boiling point increases, as the molecules have a greater surface area and electrons for stronger London forces
- The halogens have seven electrons in their outer shell (s2p5), so they need to gain one electron to obtain a stable octet, forming ions with a charge of 1-, this is shown by the reduction of chlorine:
Cl2 + 2 e^- → 2 Cl^-
- A more reactive halogen will displace the halide ion of a less reactive halogen from solution
- If a solution of chlorine (Cl2) is added to a solution of bromide (Br^-), the chlorine will displace the bromine
Cl2 + 2 Br- → 2 Cl^- + Br2
Halogen | Colour in Water |
Chlorine | Pale green |
Bromine | Orange |
Iodine | Brown |
- Precipitation reactions with aqueous silver ions are used to test for halide ions. Silver halides are formed when AgNO3 (aq) is added to a halide solution:
Ag^+ (aq) + X^- (aq) → AgX (s)
Silver Halide | Colour of Precipitate | Addition of Dilute Ammonia | Addition of Concentrated Ammonia |
AgCl | White | Dissolves to give a colourless solution | Dissolves to give a colourless solution |
AgBr | Cream | Does not dissolve | Dissolves to give a colourless solution |
Agl | Yellow | Does not dissolve | Does not dissolve |
Use of Chlorine and Chlorate
- Chlorine can dissolve in water to form hydrochloric acid and chloric(I) acid in a disproportion reaction (chlorine is both oxidised and reduced).
Cl2 (g) + H2O (l)→ HCl (aq) + HClO (aq)
Oxidation States: 0 -1 +1
- Chloric(I) acid is an oxidising agent that kills bacteria and sanitises water
- Cl2 (g) + 2NaOH (aq) → NaCl (aq) + NaClO (aq) + H2O (l)
- Sodium chlorate(I) has a bleaching action which allows it to be an active ingredient in household bleach.
- The advantages of using chlorine to treat water is that it prevents the spread of waterborne disease and sanitises water. However, it is added without direct customer consent, chlorine is a respiratory irritant and it could react to form chlorinated hydrocarbons, implicated in cancers.