Chemical Properties of Group 7 (AQA A Level Chemistry)

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Halogens: Oxidising Agents

  • Halogens react with metals by accepting an electron from the metal atom to become an ion with 1- charge

Eg.   Ca (s) + Cl2 (g) → CaCl2 (s)  consisting of Ca2+ and 2Cl- ions

  • Halogens are therefore oxidising agents:
    • Halogens oxidise the metal by removing an electron from the metal (the oxidation number of the metal increases)
    • Halogens become reduced as they gain an extra electron from the metal atom (the oxidation number of the halogen decreases)

  • The oxidising power of the halogens decreases going down the group (the halogens get less reactive)
  • This can be explained by looking at their electronegativities:

Group 17 - Electronegativity Halogens, downloadable AS & A Level Chemistry revision notes

The electronegativity of the halogens decreases going down the group

  • The electronegativity of an atom refers to how strongly it attracts electrons towards itself in a covalent bond
  • The decrease in electronegativity is linked to the size of the halogens
  • Going down the group, the atomic radii of the elements increase which means that the outer shells get further away from the nucleus
  • An ‘incoming’ electron will therefore experience more shielding from the attraction of the positive nuclear charge
  • The halogens’ ability to accept an electron (their oxidising power) therefore decreases going down the group

 

Group 17 - Trend Oxidising Power, downloadable AS & A Level Chemistry revision notes

With increasing atomic size of the halogens (going down the group) their electronegativity, and therefore oxidising power, decreases

  • The reactivity of halogens is also shown by their displacement reactions with other halide ions in solutions
  • A more reactive halogen can displace a less reactive halogen from a halide solution of the less reactive halogen
    • Eg. The addition of chlorine water to a solution of bromine water:

Cl2 (aq) + 2NaBr (aq) → 2NaCl (aq) + Br2 (aq)

    • The chlorine has displaced the bromine from solution as it is more reactive which can be summarised in the following ionic equation:

Cl2 (aq) + 2Br- (aq) → 2Cl- (aq) + Br2 (aq)

Halide Ions: Reducing Agents

  • Halide ions can also act as reducing agents and donate electrons to another atom
  • The halide ions themselves get oxidised and lose electrons
  • The reducing power of the halide ions increases going down the group
  • This trend can be explained by looking at the ionic radii of the halide ions

Group 17 - Electron Arrangement in Halide Ions, downloadable AS & A Level Chemistry revision notes

The diagram shows that going down the group the ionic radii of the halide ions increases

  • Going down the group, the halide ions become larger
  • The outermost electrons get further away from the nucleus
  • The outermost electrons also experience more shielding by inner electrons
  • As a result of this, the outermost electrons are held less tightly to the positively charged nucleus
  • Therefore, the halide ions lose electrons more easily going down the group and their reducing power increases

 

The reducing power of the halide ions increases going down the group

Examiner Tip

Halogen is a derived from a greek word meaning salt-maker, a good reminder of their reaction with metals to form salts.

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.