The Halogens
The Halogens (Group 7) consist of elements which are non-metals and have coloured vapours. They consist of molecules and form ionic salts with metals, and form molecular compounds with other non-metallic compounds. They are also in their diatomic molecule form.
Halide are halogen compounds. If the compound is with a metal, it contains halide ions
e.g. F-, Cl-, Br-, I-, At-.
e.g.
The further down an element is in group 7, its melting point and boiling point become higher and the element becomes less reactive. A more reactive halogen can displace a less reactive halogen from an aqueous solution of its ions.
Displacement Reactions
These are reaction to gain electrons. The more reactive halogen wins.
|
Halide |
Cl- (Colourless) |
Br- (Colourless) |
I- (Colourless) |
|
Halogen |
|
|
|
|
Cl2 (Green) |
No Reaction (Green) |
Br2 (Yellow) |
I2 (Brown) |
|
Br2 (Yellow) |
No Reaction (Yellow) |
No Reaction (Yellow) |
I2 (Brown) |
|
I2 (Brown) |
No Reaction (Brown) |
No Reaction (Brown) |
No Reaction (Brown) |
We see that the Halogens become less reactive down the group. They become worse at gaining electrons.
Cl2 + 2Br-
à Br2 + 2Cl-Cl2 + 2I-
à I2 + 2Cl-Br2 + 2I-
à I2 + 2Br-The group 7 atoms react by gaining an electron to form a "Noble Gas Electron Arrangement". This becomes more difficult down the group because the outer energy level is further away from the attraction of the nucleus., and more shielded by lower energy levels.
These displacement reactions are examples of REDOX reactions.
Cl2 + 2Br-
à Br2 + 2Cl-Cl2 - 2Cl- is Reduction (Gain of electrons)
2Br- - Br2 is Oxidation (Loss of electrons)
Chlorine acts as a oxidising Agent causing Bromide ions to be Oxidised.
Bromide acts as Reducing Agent causing and have been Reduced.
Reduction cannot occur without oxidation. They always occur together. This is why they are referred to as REDOX reactions.
Halogens become worse Oxidising Agents down the group.
Halide ions become better as Reducing Agents down the group.