Group 7

• Fluorine (F2)


  Very pale yellow gas, highly reactive
• Chlorine (Cl2)


  Greenish, reactive gas, poisonous in high concentrations
• Bromine (Br2)


  Red liquid, gives off dense brown/orange poisonous fumes
• Iodine (I2)


  Shiny grey solid, sublimes to purple gas
• As the molecules become larger


  They have more electrons and so have larger van der waals forces between the molecules, increasing the melting and boiling points
• As one goes down the group

  The electronegativity of the elements decreases
• As one goes down the group

  The atomic radii increases due to the increasing number of shells, making the nucleus less able to attract the bonding pair of electrons
• Displacement reactions of halide ions by halogens

  1. A halogen that is a strong oxidising agent will displace a halogen that has a lower oxidising power from one of its compounds

  2. The oxidising strength decreases down the group

  3. Oxidising agents are electron acceptors

  4. Chlorine will displace both bromide and iodide ions; bromine will displace iodide ions
• Colour of the solution

  Chlorine = very pale green solution (often colourless), Bromine = yellow solution, Iodine = brown solution (sometimes black solid present)
• Reactions of halide ions with silver nitrate

  1. The test solution is made acidic with nitric acid, and then silver nitrate solution is added dropwise

  2. Fluorides produce no precipitate

  3. Chlorides produce a white precipitate

  4. Bromides produce a cream precipitate

  5. Iodides produce a pale yellow precipitate

  6. Silver chloride dissolves in dilute ammonia, silver bromide dissolves in concentrated ammonia, silver iodide does not react with ammonia
• Reducing power of halides


  Increases down group 7 as the ions get bigger and it is easier for the outer electrons to be given away
• Reaction of halide salts with concentrated sulfuric acid

  1. F- and Cl- ions are not strong enough reducing agents to reduce the S in H2SO4, only acid-base reactions occur

  2. Br- ions are stronger reducing agents than Cl- and F- and after the initial acid-base reaction, the bromide ions reduce the sulfur in H2SO4 from +6 to + 4 in SO2

  3. I- ions are the strongest halide reducing agents, they can reduce the sulfur from +6 in H2SO4 to + 4 in SO2, to 0 in S and -2 in H2S
• Chlorine is used in water treatment to kill bacteria, the benefits to health outweigh its toxic effects
• Disproportionation reactions of chlorine

  1. Cl2 (g) + H2O (l) ⇌ HClO (aq) + HCl (aq)

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

  3. Cl2 (aq) + 2 NaOH (aq) NaCl (aq) + NaClO (aq) + H2O (l)
• Naming chlorates/sulfates


  In IUPAC convention the various forms of sulfur and chlorine compounds where oxygen is combined are all called sulfates and chlorates with relevant oxidation number given in roman numerals