AQA AS Chemistry Unit 2: Halogens and Halide Ions notes

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Group 7: The Halogens and Halide Ions Notes

Learning Objectives:

  • Understand the trends in electronegativity, atomic radium and boiling points of the halogens.
  • Understand that the ability of the halogens (from fluorine to iodine) to oxidise decreases down the group (e.g. the displacement reactions with halide ions in aqueous solution).
  • Know the reaction of chlorine with cold, dilute, aqueous NaOH and the uses of the solutions formed.
  • Know the reactions of chlorine with water and the use of chlorine in water treatment.
  • Appreciate that the health of water treatment of chlorine outweigh its toxic effects.
  • Understand the trend in reducing ability of the halide ions.
  • Know the different products formed by reaction of NaX and H2SO4.
  • Understand why acidified silver nitrate solution is used as a reagent to identify and distinguish between F-, CI -, Br and I -

Trends of Boiling Point, Electronegativity and Atomic Radius:

Atomic Radius:

The atomic radius increases as you go do the group because there are more electron shells between the nucleus and the bonding electrons to shield the nuclear charge.

Boiling Points:

The boiling points increase as you go down the group because there are van der Waals forces between the molecules and there is increasing strength of the van der Waals forces as the size and relative mass of the atoms increases. Also, there are more electrons.


The power of an atom to attract shared pair of electrons in a covalent bond. The electronegativity decreases down the group because large atoms attract shared electrons less than smaller ones. So going down the group, as the atoms become larger, the electronegativity decreases.

Displacement Reactions:

Displacement reaction - Is one in which a more reactive metal displaces a less reactive metal.

  • When Halogens react they gain (minus charge) an electron. This means that they are oxidising agents.
  • Get less reactive down the group because the atoms get larger (and less electronegative).
  • A halogen will displace a halide ion from a solution.

If you add chlorine to a solution containing bromide ions e.g. potassium bromide, it will displace the bromine - and there


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