Group 7 - The Halogens

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Halogens are the Highly Reactive Non-Metals of Group 7

Their boiling points increase down the group. This is due to the increasing strength of the van der Waals forces as the size and the relative mass of the molecules increases. this trend is shown the the changes of physical state from fluorine (gas) to iodine (solid).

Electronegativity decreases down the group. Electronegativity is the tendency of an atom to attract a bonding pair of electrons. The halogens are all highly electronegative elements. But larger atoms attract electrons less than small ones. This is because the electrons are further from the nucleus and are shielded by more electrons.

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Halogens Displace Less Reactive Halide Ions from Solution

When the halogens react, they gain an electron. they get less reactive down the group because the atoms become larger. The outer shell is further from the nucleus, so electrons are less strongly attracted to it. So you can also say that the halogens become less oxidising down the group.

The relative oxidising strengths of the halogens can be seen in their displacement reactions with other halide ions. The basic rule is: A halogen will displace a halide from solution if the halide is below it in the periodic table.

These displacment reactions can be used to identify which halogen (or halide) is present in a solution.

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Chlorine and Sodium Hydroxide Make Bleach

If you mix chlorine gas with cold, dilute aqueous sodium hydroxide you get sodium chlorate(I) soultion, NaClO(aq), which is household bleach (which kills bacteria).

2NaOH(aq) + cl2(g) ------> NaClO(aq) + NaCl(aq) +H2O(l)

In this reaction, the oxidation state of Cl both increases to +1 and decreases to -1, making it a disproportionation reaction.

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Chlorine is used to Kill Bacteria in Water

When you mix chlorine with water, it undergoes disproportionation. You end up with a mixture of chloride ions and chlorate(I) ions.

In sunlight, chlorine can also decompose water to form chlorine ion and oxygen.

Chlorate(I) ions kill bacteria. So, adding chlorine to water can make it safe to drink or swim in. On the downside, chlorine is toxic.

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 can be very serious or fatal.

Water contains a variety of organic compounds, e.g. from the decomposition of plants. Chlorine reacts with these compounds to form chlorinated hydrocarbons, e.g. chloromethane and many of these chlorinated hydrocarbons are carcinogenic. However, this increased cancer risk is small compared to the risks of untreated water - a cholera epidemic could kill thousands of people.

We have to weigh up these risks and benefits when making decisions about whether we should add chemicals to drinking water supplies.