AS-Level Group 7 (Halogens)

  • Halogen atoms gain one electron to have a full outer shell – they are reduced.

  • For a halogen atom to gain an electron, another species must lose an electron (be oxidised). 

  • Halogens are oxidising agents as they can force other species to be oxidised.

  • Oxidising agents are reduced. 

  • The ability of halogens to act as oxidising agents decreases down the group.

    • Fluorine is the strongest oxidising agent of the halogens.

  • Halogens can oxidise less reactive halide ions in displacement reactions.

QUICK NOTES

Oxidising Power of the Halogens

 

The halogens have seven electrons in their outer shell, so they gain one electron to have a full outer shell (eight electrons).

 

X + e⁻ → X⁻

 

By gaining an electron they are reduced, but they need to get this electron from somewhere! In order to gain an electron, another species must lose an electron. The species the halogen gets the electron from is oxidised. Because of this, we describe the halogens as oxidising agents, as they cause another species to be oxidised.

 

Oxidising agents are reduced!

 

As you go down group 7, the strength of the halogens as oxidising agents decreases. The number of electron sub-shells increases, meaning that the outermost electrons have more inner sub-shells between them and the nucleus.

 

 

 

 

 

 

 

 

 

 

These extra inner sub-shells effectively dilute the positive charge from the nucleus that reaches the outermost electrons (increased shielding). This results in the positive charge being weaker at the surface of the atom. A weaker positive charge at the surface means the atom is less able to attract an electron from another species. It cannot act as an oxidising agent as well as the halogens above it in the group.

 

Fluorine is the strongest oxidising agent, so it’s very reactive.

 

Displacement Reactions

 

As some of the halogens are stronger oxidising agents than others, they can actually oxidise halogens less reactive than themselves.

 

In solution, chlorine is able to oxidise both bromide ions and iodide ions as it ‘wants’ an extra electron more than the Br and I. The chlorine atoms basically steal electrons from the ions.

 

Cl (aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br (aq)

Cl (aq) + 2I⁻ (aq) → 2Cl⁻(aq) + Br (aq)

 

Bromine is able to oxidise iodide ions, but not chloride or fluoride ions.

 

Br (aq) + 2I⁻(aq) → 2Br⁻(aq) + I (aq)

 

The products made in such reactions are very easy to identify, as the halogen molecules have different colours.

 

Cl  = pale green

Br  = orange

I  = brown / purple

2

2

2

2

2

2

2

2

2