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
Worked Example
Which of the statements below are correct?
I. Potassium chloride solution will react with fluorine to form chlorine.
II. Sodium chloride solution will react with iodine to form chlorine.
III. Lithium iodide solution will react with bromine to form iodine.
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
Answer
The correct option is B
I is correct
Fluorine is above chlorine in Group 17 and will displace it
II is incorrect
Iodine is below chlorine and cannot displace it
III is correct
Bromine is above iodine and displaces it from lithium iodide
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