Group 7

• Fluorine forms a pale yellow gas
• Chlorine forms a pale green gas
• Bromine forms a brown-orange liquid
• Iodine forms a grey solid
• Boiling points increases as we go down the group because the van der Waals forces increase due to the increasing size and relative mass of the atoms

  The physical state goes from gas at the top of group 7 to solid at the bottom
• Electronegativity decreases as we go down the group

  Electronegativity is the ability for an atom to attract a pair of electrons in a covalent bond towards itself

  The atoms get larger and the distance between the positive nucleus and bonding electrons increases, there's also more shielding'
• Displacement reactions


  More reactive halogens will displace less reactive halide ions

  Reactivity in halogens decreases as we go down group 7, for a reaction to occur an electron is gained

  Atoms with a smaller radius attract electrons better than larger atoms

  Halogens are less oxidising as we go down the group, we can show this by reacting halogens with halide ions
• Bleach


  Made via a disproportionation reaction

  Mixing chlorine and sodium hydroxide forms sodium chlorate(I) solution- aka bleach

  2NaOH + Cl2 --> NaClO + NaCl + H2O

  Cl2 has an oxidation state of 0, chlorine in NaClO has an oxidation state of -1 and chlorine in NaCl has an oxidation state of +1- chlorine has been simultaneously reduced and oxidised, disproportionation reaction
• Uses of chlorine


  Water sterilisation

  Adding water to chlorine will produce chlorate(I) ions (ClO-) which kill bacteria- useful in drinking water and pools

  H2O + Cl2 --> 4H+ + 2Cl- + O2

  However, sunlight can decompose chlorinated water but NO ClO- is made

  2H2O + Cl2 --> 4H+ + 2Cl- + O2
• What are the advantages of chlorinating drinking water?

  Destroys microorganisms that cause disease

  Long lasting so reduces bacteria build up further down the supply

  Reduces the growth of algae that discolours water and can give it a bad smell and taste
• What are the disadvantages of chlorinating drinking water?

  Chlorine gas is toxic and irritates the respiratory system

  Liquid chlorine causes severe chemical burns to the skin

  Chlorine can react with organic compounds present in the water to make chloroalkanes- these have been linked with causing cancer, however the risk of not chlorinating water could lead to a cholera epidemic
• Reducing power of halide ions

  As we go down the group the ionic radius increases

  The distance between the nucleus and outer electrons becomes larger and there is more shielding- the attractive force gets weaker

  The outer electron is lost more readily and this is the reason why I- is a more powerful reducing agent than F-
• Halide ions with silver nitrate

  Testing for Cl-, Br- and I- involves adding dilute nitric acid then silver nitrate. The colour of the precipitate will help you identify the halide ion

  We add nitric acid to react with any anions other than halides e.g carbonates, this could give a false result

  • Cl-= white precipitate forms Ag+ + Cl- --> AgCl
  • Br-= cream precipitate forms Ag+ + Br- --> AgBr
  • I-= yellow precipitate forms Ag+ + Br- --> AgI

  Further test: add NH3

  • Cl-= white precipitate redissolves with dilute NH3
  • Br-= cream precipitate redissolves in concentrated NH3
  • I-= no redissolving
• Tests for ions


  Must be in a specific order to prevent false positives

  1. test for carbonates
  2. test for sulfates
  3. test for halides