The Halogens

• They are electronegative elements.
• They have a typical oxidation number of -1.
• They are usually the oxidising agents in a reaction as they receive electrons becoming reduced.
• The tendency to form anions decreases down the group.
• Fluorine is the most electronegative element.
• The melting point increases down the group from Fluorine to Iodine due to the increasing intermolecular forces holding the diatomic elements together
• This increase is due to the increasing number of electrons in the molecules contributing to the induced dipole-induced dipole intermolecular force. 
• Halogen + Metal --> Metal Halide
• Reactivity decreases down the group from Fluroine to Iodine.

This test will allow you to identify whether a halide ion is a Chloride, Bromide or Iodide ion.

• Add Silver Nitrate 
• The Silver Halide is then precipitated 
• This is because Silver halide are insoluble in water

The results look like this:

• Silver Chloride = White ppt
• Silver Bromide = Cream ppt
• Silver Iodide = Pale yellow ppt

Chlorine

• Chlorine is used in drinking water as it kills pathogenic bacteria and viruses.
• Which prevents the outbreak of serious diseases e.g. Typhoid and Cholera.
• It is applied in its gaseous state to water;
• Cl2 + H2O --> HOCl + HCl

Fluorine

• It is added to both water and toothpaste.
  
• It only has beneficial effects below 1ppm*
• It reduces tooth decay cause by cavity formation
• and in strengthening bones, thus reducing osteoporosis.
• It is added as flurosilicic acid, sodium fluorosilicate or sodium fluoride.

*1 ppm= one part per million.