Tuesday, 9 February 2016

Group VII - The Halogens

What are the elements?

The elements in Group VII are fluorine, chlorine, bromine, iodine and astatine (radioactive).
Chlorine, bromine and iodine are the only elements in this group that you need to have an in depth knowledge of.

Properties of chlorine, bromine and iodine

  • They exist as diatomic molecules, e.g Br₂
  • Their colour darkens as you move down the group - chlorine is pale green, bromine is red-brown and iodine is purple-black
  • They form molecular compounds with other non-metals, e.g. HCl
  • As you move down the group they gradually change state, chlorine is a gas, bromine is a liquid and iodine is a solid.
  • When they react with hydrogen they produce hydrogen halides, for example HCl,  and hydrogen halides produce acidic solutions when dissolved in water, for example HCl becomes hydrochloric acid
  • When they react with metals they produce ionic metal halides, 2FeCl₃ (iron(III) chloride) is an ionic metal halide

Electronic structure and reactivity

The reactivity decreases as you go down the group, this is unlike what happens in Groups I and II, the order of reactivity is below:
Image courtesy of http://isbchem1.pbworks.com

When the halogens react, they gain an extra electron. This is because they have 7 electrons in their outer shell (we know this because they're in Group VII) so to fill their outer shell it's easiest to gain 1 electron, rather than loose 7. When that 1 extra electron, they then have the electron configuration of a noble gas.

The reason chlorine is more reactive than iodine is because chlorine is a smaller atom, so the electrons in the outer shell are held in closer to the nucleus. As you go down the group, the outer electron shell will get further away and will be held less securely.

Uses of fluorine, chlorine, bromine and iodine

Florine - used for toothpaste and in drinking water
Chlorine - kills bacteria in drinking water, an ingredient in PVC plastic and is in bleach
Bromine - used in medicines, fire retardants and disinfectants
Iodine - used in medicines, disinfectants and is also used as a photographic chemical

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