Group 7 Elements



Atomic Radii

  • Increase down the group
  • This is due to extra shells of electrons being added at increasing distance from the nucleus. Increased shielding furthur weakens the attraction of the outer electrons to the nucleus and so increases their distance. These factors outweigh the increasing nuclear charge


  • Decreases down the group
  • This is because extra shells of electrons are added at increasing distance from the nucleus and with increased shielding. These factors outweigh the effect of increasing nuclear charge, so attraction of the bonding electrons to the nucleus decreases down the group

Structure and Bonding

Elements: Halogens all have simple molecular structures made up of diatomic molecules with a single bond between the two atoms

Ionic Compounds: Halogens all form ionic compounds . They form negative ions called halide ions by gaining one electron

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  • The halogen molecules do not have a permanent dipole, so the only intermolecular forces are van der Waals forces. These become increasingly strong on going down the group as the number of electrons per molecule increases. That means on going downt the group;
  • The energy needed to vaporise the molecules increases
  • The volatility decreases
  • The boiling point increases

Appearance at room temperature

  • Chlorine is a green/yellow gas
  • Bromine is a red-brown liquid giving off brown vapour
  • Iodine is a gred/black solid
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1. Displacement Reactions

  • On going down the group the ability of the halogens to gain an extra electron decreases
  • These reactions are redox reactions
  • Displacement reactions are accompanied by colour changes, because the halogens are coloured but the halide ions are not. The colours are most clearly seen by adding an organic solvent such as cyclohexane in which the neutral halogen molecules readily dissolve and show their characteristic colours
  • The reason for the decreased reactivity of the elements on going down the group is that more and more shells are being added furthur from the nucleus and the shielding by inner electrons also increases. These two factors outweigh the increasing nuclear charge so that the nucleus becomes less able to attract an extra electron into the outer shell

Colours in cyclohexane

Bromine: Orange

Chlorine: Yellow

Iodine: Purple

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2. Testing for halide ions by reaction with aqueous silver ions

  • If aqueous silver ions (in aqueous silver nitrate) are added to an aqueous solution of chloride, bromide of iodide a precipitate of the corresponding silver halide is formed
  • A chloride gives a white precipitate of silver chloride is formed
  • A bromide gives a cream precipitate of silver bromide
  • An iodide gives a yellow precipitate of silver iodide

Slubility of silver halides in aqueous ammonia

  • A furthur test can be carried out to see if the precipitate dissolves in aqueous ammonia;
  • Silver chloride dissolves in dilute (and conentrated) aqueous ammonia
  • Silver bromide dissolves in concentrated (but not dilute) aqueous ammonia
  • Silver iodide is insoluble in dilute and concentrated aqueous ammonia
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3. Reactions of chlorine with water and cold dilute aqueous sodium hydroxide

Reaction with water

  • In aqueous chlorine a small proportion of the chlorine molecules react with water to form a mixture of hydrochloric acid and chloric(1) acid
  • In this reaction half of the chlorine is reduced and the other half is oxidised. A reaction of this sort in which the same element is simultaneously oxidised and reduced is called a disproportionation reaction

Chlorination in water treatment

  • Chloric(1) acid kills bacteria and this is why aqueous chlorine is used in water treatment to prevent life threatening diseases
  • However, some people have concerns about the use of chlorine because;
  • Chlorine itself is highly toxic
  • Chlorine can also react with traces of organic compounds in water producing organochlorine compounds, some of which are know carcinogens
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Reaction with aqueous sodium hydroxide

  • Chlorine also undergos a disproportionation reaction with aqueous sodium hydroxide to form sodium chloride and sodium chlorate(1)
  • Domestic bleach contains aqueous sodium chlorate(1) and sodium chloride made in this way. It is used to bleach clothing etc by removing stains and also as a disinfectant
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