group 7

• Group 7 elements


  • Highly reactive non-metals
  • Need to gain an electron to form a 1- ion and achieve a full outer shell of electrons
• Atomic radius of group 7 elements

  Increases down the group due to additional electron shells
• Reactivity of group 7 elements

  Decreases down the group as it becomes harder to attract an electron due to increased shielding and larger atomic radius
• First ionisation energy of group II metals


  Decreases down the group due to a greater atomic radius and increased amounts of shielding
• Boiling point of group 7 elements

  Increases down the group as the strength of the van der waals forces increases due to larger atomic radius
• Fluorine is a gas at room temperature whereas iodine is a solid
• Oxidising power of halogens


  Decreases down the group as their ability to attract electrons decreases due to shielding and a greater atomic radius
• Reducing power of halide ions

  Increases down the group as electrons are easier to lose from larger ions due to shielding and a larger atomic radius
• Reactions of halide ions with H2SO4

  1. Fluoride and chloride ions

  2. Bromide ions

  3. Iodide ions
• Acidified silver nitrate


  Used to test for halide ions as it reacts to form different coloured precipitates
• Precipitates formed with silver nitrate

  • Cl- (white)
  • Br- (cream)
  • I- (yellow)
• Further testing of precipitates with ammonia

  1. Cl- (precipitate dissolves)

  2. Br- (no change)

  3. I- (no change)
• Reaction of chlorine with cold water

  Produces chlorate(I) ions (ClO-) and chloride ions
• This is a disproportionation reaction as the chlorine is both oxidised and reduced
• Reaction of chlorine with water in the presence of UV light

  Produces oxygen and hydrochloric acid
• Chlorine is used in small quantities to kill bacteria in water treatment processes, but it can be toxic
• Production of bleach


  Chlorine can be mixed with cold, aqueous sodium hydroxide to produce sodium hypochlorite
• Testing for halide ions with acidified silver nitrate and ammonia

  1. Cl- (white precipitate, dissolves in conc. NH3)

  2. Br- (cream precipitate, no change with NH3)

  3. I- (yellow precipitate, no change with NH3)
• Testing for sulfate ions (SO4 2-)

  Using BaCl2 which reacts to form a white precipitate
• Testing for hydroxide ions (OH-)

  Using red litmus (turns blue) or universal indicator (turns blue-purple)
• Testing for carbonate ions (CO3 2-)

  Adding acid (HCl) causes effervescence and CO2 gas is given off, which turns limewater cloudy
• Flame tests for group 2 cations

  1. Ca2+ (brick red)

  2. Sr2+ (red)

  3. Ba2+ (pale green)
• Testing for ammonium ions (NH4+)

  1. Holding red litmus over the substance, which turns blue if ammonium ions are present

  2. Adding NaOH to produce ammonia gas faster