Highly Reactive Non-Metals of Group 7
The table below gives some of the main properties of the first 4 halogens:
1s2 2s2 2p5
↑Increases up the group
1s2 2s2 2p6 3s2 3p5
1s2 2s2 2p6 3s2 3d10 4s2 4p5
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6
Reasons for boiling points and electronegativity
1. Their boiling points increase down the group
This is due to the increasing strength of the Van der Waals forces as the size and relative mass of the atoms increases. This trend is shown in the changes of physical state from fluorine (gas) to iodine (solid).
2. Electronegativity decreases down the group
Electronegativity, remember, is the tendency of an atom to attract a bonding pair of electrons. The halogens are highly electronegative elements. But larger atoms attract electrons less than smaller ones. So, going down the group, as the atoms become larger, the electronegativity decreases.
1. When the halogens react, they gain an electron. They get less reactive down the group, because the atoms become larger (and less electronegative). So you can say that the halogens become less oxidising down the group.
2. The relative oxidising strengths of the halogens can be seen in their displacement reactions with the halide ions. A halogen will displace a halide from solution if the halide is below it in the periodic table.
3. These displacement reactions can be used to help identify which halogen (or halide) is present in a solution.
If you mix chlorine gas with dilute sodium hydroxide at room temperature, you get sodium chlorate (I) solution, NaCl, which just happens to be common household bleach.
2NaOH + Cl2 ---> NaClO + NaCl + H2O
0 +1 -1
(The oxidation state of Cl goes up and down. This is disproportionation)
The sodium chlorate (I) solution (bleach) has loads of uses - it's used in water treatment to bleach paper and textiles... and it's good for cleaning toilets, too. Handy...
Killing Bacteria with Chlorine
When you mix chlorine with water, it undergoes disproportionation. You end up with a mixture of hydrochloric acid and chloric (I) acid (also called hypochlorous acid).
Cl2 + H2O ---> HCl + HClO
0 -1 +1
Aqeous chloric (I) acid ionises to make chlorate (I) ions (also called hypochlorite ions). Chlorate (I) ions kill bacteria.
HClO + H2O ---> ClO- + H3O-
So adding chlorine (or a compound containing chlorate ions) to water can make it safe to drink or swim in.
In the UK our drinking water is treated to make it safe. Chlorine is an important part of water treatment:
- It kills disease-causing microorganisms
- Some chlorine persists in the water and prevents reinfection further down the supply.
- It prevents the growth of algae, eliminating bad tastes and smells, and removes discolouration caused by organic compounds.
However, there are risks from using chlorine to treat water:
- Chlorine gas is very harmful if it's breathed in - it irritates the respiratory system. Liquid chlorine on the skin or eyes causes severe chemical burns. Accidents involving chlorine could be really serious, or fatal.
- Water contains a variety of organic compounds, e.g. from the decomposistion of plants. Chlorine reacts with these compounds to form chlorinated hydrocarbons, e.g. chloromethane - and many of these chlorinated hydrocarbons are carcingenic (cancer- causing). However, this increased cancer risk is small compared to the risks from untreated water - a cholera epidemic, say, could kill thousands of people.