M - FROM MINERALS TO ELEMENTS
Introduction
Oxidation and Reduction
Group 7 Elements: The Halogens 
Theory Of Solutions
Extracting Copper from its Ore
Acid-Base Theory
Introducing The Halogens
Chemical Reactions of the Halogens
Solubility of the Halogens
Indroducing The Halogens
| ELEMENT | SYMBOL | ELECTRONIC CONFIGURATION | APPEARANCE |
| Fluorine | F | [He] 2s22p5 | Green-yellow gas |
| Chlorine | Cl | [Ne] 3s23p5 | Green-yellow gas |
| Bromine | Br | [Ar] 3d104s24p5 | Dark red liquid | Iodine | I | [Kr] 4d105s25p5 | Grey-black solid |
| Astatine | At |
Generally speaking, the halogens are the most reactive group of non-metals. They are powerful oxidising agents compared with Group 1 and Group 2 metals which are reducing agents. They all have outer electronic configurations of s2p5. Therefore one electron is required to completely fill the p orbital and attain the noble gas configuration. The tendency is to form negatively charged ions.
X + e- X-
Fluorine will most readily accept this electron, i.e. most electronegative, because its outer electrons are nearest to the nucleus; iodine will accept the electron much less readily. Fluorine is therefore the most reactive element in the group. The halogen atoms can form ionic bonds with metals. They can also form covalent bonds with non-metals. The halogens exist as simple diatomic molecules, e.g. Cl2.
The halogens are so reactive they cannot exist free in nature. They are usually found combined with metals. Fluorine is found as fluorspar CaF2 (Blue John) and cryolite Na3AlF6. Chlorine is found as sodium chloride. Bromides and iodides are found in sea water.
Both fluorine and chlorine are obtained by electrolysis. Bromine is obtained from the bromide in sea water by oxidation with chlorine. Chlorine can be made by the electrolysis of a concentrated solution of sodium chloride. The other products are hydrogen and the alkali sodium hydroxide. Three well-established technologies have been established for electrolysing brine:
- Mercury cell.
- Diaphragm cell.
- Membrane cell.
The half-equations involved are:
| at the anode: | 2Cl-(aq)
Cl2(g) + 2e- |
| at the cathode: | 2H2O(l) + 2e-
2OH-(aq) + H2(g) |
Sodium ions from the sodium chloride are able to combine with the hydroxide ions to form sodium hydroxide.
Chemical Reactions of the Halogens
- Reactions with Metals
The halogens are very reactive with metals, the vigour of the reaction depending upon the position of the metal in the activity series and the particular halogen. In all cases the halide is formed.
E.g. 2Na + Cl2 2NaCl2Fe + 3Cl2 2FeCl32Al + 3I2 2AlI3 - Reactions with Non-metals
The halogens also react with non-metals. Fluorine even reacts with diamond and xenon.
C + 2F2 CF4
Chlorine reacts violently with phosphorus.
2P + 3 Cl2 2PCl3
All the halogens react with hydrogen to form hydrogen halides.
H2(g) + Cl2(g) 2HCl(g) - Displacement Reactions
As chlorine is a more powerful oxidising agent than bromine, chlorine will oxidise bromide ions to bromine.
Cl2(aq) + 2 Br-(aq) 2Cl-(aq)
+ Br2(aq)
Similarly chlorine will oxidise iodide ions to iodine.
Cl2(aq) + 2I-(aq) 2Cl-(aq) +
I2(aq)
Bromine oxidises iodide ions to iodine.
Br2(aq) + 2I-(aq) 2Br-(aq) +
I2(aq) - Reactions with Silver Ions
Solutions of halides give precipitates with aqueous silver nitrate.
Ag+(aq) + X-(aq) AgX(s)
The colour of the precipitate is an indication of the halide:
Silver chloride White Silver bromide Cream Silver iodide Yellow
Silver chloride and silver bromide are decomposed by light to produce silver and the halogen:
2AgBr(s) 2Ag(s) + Br2(l)
This reaction is the basis of most photographic processes.
Solubility of the Halogens
The halogens are sparingly soluble in water. Chlorine and bromine produce a mixture of acids with water. The solutions act as a bleach.
Cl2(g) + H2O(l) HCl(aq) + HClO(aq)
Iodine is almost insoluble.
The halogens are more soluble in non-polar solvents such as hexane. They produce characteristic coloured solutions, e.g. bromine - orange, iodine - pink.
