Group 17 Elements with Halide Ions (DP IB Chemistry): Revision Note

Group 17 Elements with Halide Ions

• The halogens are the Group 17 non-metals

  • They are poisonous

  • They include fluorine, chlorine, bromine, iodine and astatine

• Halogens are diatomic

  • This means that they form molecules of two atoms

• All halogens have seven electrons in their outer shell

  • They form halide ions by gaining one more electron to complete their outer shells

Colours and states at room temperature

Halogen colours and states at room temperature:

• Fluorine: yellow gas

• Chlorine: pale green gas

• Bromine: orange-brown liquid that readily evaporates to form a brown gas

• Iodine: grey-black solid that sublimes to form a purple vapour

Colour of halogen solutions in water:

• Fluorine: not typically tested due to its reactivity

• Chlorine: green-blue solution

• Bromine: orange solution

• Iodine: dark brown solution

Physical properties of the halogens

• The melting and boiling points of the halogens increase down the group

• This is due to increasing molecular mass and stronger van der Waals forces between molecules

Reactivity trend in Group 17

• The reactivity of the halogens decreases down the group

• Each halogen has an outer electron configuration of ns²np⁵

  • This means that they react by gaining one outer electron to achieve a full outer shell

• Descending the group:

  • The atomic radius increases

  • The number of electron shells increases, causing shielding to increase

  • Electron affinity becomes less negative (less exothermic)

• This results in:

  • The electrostatic forces of attraction between the nucleus and any incoming electron is weaker

  • So, the halogens become less reactive down the group

• This decrease in reactivity reflects the decreasing non-metallic character of the elements

  • They become less effective at gaining electrons

Displacement reaction of the halogens with halide ions

• A more reactive halogen will displace a less reactive halogen from an aqueous solution of its halide

• The order of reactivity among chlorine, bromine and iodine is:

  • Chlorine > Bromine > Iodine

Example: chlorine displaces bromine

• If you add pale green chlorine solution to colourless potassium bromide solution, the solution becomes orange as bromine forms

• Chlorine is above bromine in Group 17 so it is more reactive

• Chlorine will therefore displace bromine from an aqueous solution of a metal bromide

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

chlorine + potassium bromide → potassium chloride + bromine

Example: Bromine displaces iodine

• If you add orange bromine solution to colourless sodium iodide solution, the solution becomes brown as iodine forms

• Bromine is above iodine in Group 17 so it is more reactive

• Bromine will therefore displace iodine from an aqueous solution of a metal iodide

Br₂ (l) + 2NaI (aq) → 2NaBr (aq) + I₂ (aq)

bromine +  sodium iodide → sodium bromide + iodine