Specification Point 2.7:
Understand how displacement reactions involving halogens and halides provide evidence for the trend in reactivity in Group 7

Halogens & Halides Displacement Reactions

Displacement Reaction: When a more reactive Halogen displaces a less reactive halogen for an aqueous solution of its halide.

  • Reactivity of Group 7 non-metals increases as you go up the group
  • Out of the 3 halogens, chlorine, bromine and Iodine, chlorine is the most reactive and iodine is the least reactive
Group 7 reactivity trend, Edexcel IGCSE Chemistry

Aqueous Solution Colour of Halogens:

Halogen aqueous solution colour, Edexcel IGCSE Chemistry


If you add chlorine solution to colourless potassium solution, the solution becomes orange as bromine is formed. Chlorine is above bromine in group 7 so is more reactive. Chlorine will therefore displace bromine from an aqueous solution of metal bromide:

Chlorine  +  Potassium Bromide Solution  →  Potassium Chloride Solution  +  Bromine

2KBr (aq) + Cl2 (aq) → 2KCl (aq) + Br2(aq)

Bromine is above Iodine in Group 7 so is more reactive. Bromine will therefore displace Iodine from an Aqueous Solution of Metal Iodide:

Bromine  +  Magnesium Iodide Solution  →  Magnesium Bromide Solution  +  Iodine

Br2 (l) + 2MgI (aq) → 2MgBr (aq) + I2 (aq or s)

Specification Point 2.8C (Paper 2C Only):
Explain the trend in Reactivity in Group 7 in terms of Electronic Configurations

Electronic Configuration of Elements in Group 7

Group 7 electronic configurations, CIE IGCSE Chemistry

Trend in Reactivity of Group 7:

  • Reactivity of group 7 non-metals increases as you go up.
  • Each outer shell contains seven electrons and when group 7 metals react, they will need to gain one outer electron to get a full outer shell of electrons.
  • As you go up group 7, the number of shells of electrons decreases (period number decreases up the Periodic Table).
  • This means that the outer electrons are closer to the nucleus so there are stronger electrostatic forces of attraction that attracts the extra electron needed.
  • This allows an electron to be attracted more readily, making it more reactive as you go up the group.

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.