Group 7 - The Halogens

The Halogens

  • Halogens are non-metals withlow melting an d boiling points in comparison with the metals.
  • They are diatomic molecules held together by a covalent bond.
  • Increasind boiling point down a group
  • Increasing strength of London Forces between temporary dipoles down the group
  • Down the group, more energy is needed to separate molecules.
  • The molecules contain atoms of the same element so must be non-polar.
  • There are relatively weal intermolecular attractiobs between temporary dipoles
  • Melting and boiling points increase down the group as the molecules start to get larger.
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Redox Reactions/Reactivity

  • Atoms of the elements all have s2p5 outer level electron configuration so are classed a p-block elements
  • They react in many redox reactions by gaining one electron to form 1- ions
  • They are oxidising agents as they tend to gain electrons
  • Since electrons are gained, this is reduction. Halogen atoms gain 1e- to form halide ions. The electron configuration of the halide ion is now s2p6.

Down the group, there is:

  • decrease of reactivity
  • Decrease in strength as an oxidsing agent
  • Increase in size and atomic radius
  • Increase in shielding by inner shells
  • Less attraction for an additional electron
  • Decreasing ease in forming -1 ions
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Reactions

All halogens are at least partially soluble in water, though it is difficult to identify aqueous solutions of Br2 and I2 from their colour. With an organic solvent (such as cyclohexane) there producue distinctive colours:

  • F2: Pale yellow/green gas. Pale yellow/green in water. Colourless in cyclohexane.
  • Cl2: Pale green gas. Pale green in water. Pale green in cyclohaxane
  • Br2: Orange liquid. Orange in water. Orange in cyclohexane.
  • I2: Black solid. Brown in water. Violet in cyclohexane.
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Displacement

In a displacement reaction, a more reactive halogen will displace a less reactive halogen from its halide solution.

  • Flourine displaces all of the other halognes from their halides becuase they are all less reactive.
  • Chlorine displaces bromine from bromide and iodine from iodide
  • Bromine displaces less reactive iodine from iodide.
  • Iodine is not a strong enough oxidising agent to displace any of the halogens above it in Group 7 from their halides.
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Uses and Hazards of Chlorine

  • Cl2 is a toxic gas; when dissolved in water, Cl2 is a powerful disinfectant that kills harmful bacteria. It is often added in small amounts to drinking water to make it safer to drink.
  • This has made a massive improvement to public health around the world, particularly in reducing the water borne disease cholera.
  • However, environmentalists suspect that Cl2 in the water reacts with organic matter forming small amounts of chlorinated hydrocarbons and that these may cause cancer.
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