Group 7 is a set of very reactive non-metallic elements called halogens. The word halogen comes from Greek words meaning ‘salt formers’. The halogens are fluorine, chlorine, bromine and iodine. There is also a fifth member of the halogens which is not often mentioned. It is called astatine.

The halogens possess various characteristics which are summed up in the following points:

1) They are diatomic elements; which means they have two atoms per molecule.

2) They are coloured; for example, chlorine and fluorine are green gases, bromine is a brown liquid, and iodine is a grey solid which forms a purple vapour on heating.

3) The vigor of the reactions with the halogens decreases down the group. Fluorine is the most reactive. It reacts vigorously with sodium to form sodium chloride and will displace all of the other halogens from their ionic compounds(displacement reactions explained later).

4) The halogens have strong smells.

** An interesting fact is that bromine takes its name from a Greek word meaning ‘stink’.

We will use the metal sodium in our example.

The reaction between sodium and chlorine is very vigorous and the product is sodium chloride. The following equation illustrates this reaction.

sodium + chlorine = sodium chloride

2 Na(s) + Cl_{2(g) =} 2Na ⁺ Cl^-(s)

** Some of you may be wondering how the reaction between a poisionous gas(chlorine) and a very reactive metal(sodium) can result in an edible product(sodium chloride - common salt). Well for all those interested here is the reason.

On reacting chlorine with sodium the chlorine has accepted one electron from a sodium atom. The atoms of the elements have changed into ions therefore the change in the number of electrons has had a desirable effect on the properties of the atoms. Hence it is obvious that a chemical reaction changes the properties of the atoms of the reactants.

** Chlorine is a choking, toxic, oxidizing agent which turns moist litmus white. It has a bleaching effect and is used to make bleach, in water treatment to kill bacteria, and in making pesticides.

Halogens do react with water, however the reaction is not a vigorous one . However there are exceptions to this statement. Fluorine reacting with water is strong.

The remaining halogens react to decreasing extents to form an acidic, bleaching solution. The equation for the reaction of chlorine with water is below:

chlorine + water = hydrochloric acid + chloric (I) acid(bleach)

Cl₂(g) + H₂ O(l) = HCL(aq) + HOCL(aq)

The compounds formed in the reactions between halogens and non-metallic elements are molecular compounds, while on reacting halogens with metals, ionic compounds are formed.

The following reaction shows what happens when hydrogen burns in chlorine:

Cl-Cl(g) + H-H(g) = 2H-Cl(g)

The reaction between chlorine and hydrogen gives 2 molecules of the colourless gas hydrogen chloride.

First we need to discuss the trends in the reactivity of the halogens. The trend is that as you descend the group, the reactivity of the halogen decreases. Therefore fluorine is the most reactive of the halogens.

Why does this trend exist?

The explaination is relatively simple. It is to do with electrons. The aim of any halogen is to obtain a full outer shell(8). Therefore through ionic and covalent bonding this can occur, by the transfer and sharing of a/an electron(s) respectively.

The electron which is accepted is most strongly held if it enters the electron shell which is closest to the strong charge from the nucleus. Therefore the element which has its outer shell electrons closest to the positively charged nucleus is the most reactive. This means that fluorine is the most reactive of the halogens. The reactivity decreases as you descend the group as the outer shell of electrons moves further away from the nucleus which attracts electrons.

Knowing the above information on reactivity allows us to go on to explaining the displacement reactions of the halogens.

*A more reactive halogen can displace a less reactive halogen from its salt.

Here is an example of a displacement reaction which takes place in group 7:

Cl₂(g) + 2NaBr(aq) = 2NaCl (aq) + Br₂(aq)

chlorine + sodium = sodium bromine

bromide chloride

In this reaction because chlorine is in a higher position in the group than bromine, chlorine is powerful enough to displace the bromine from its salt.

Inorder to test which halide is present in a reaction, you can observe the colour of the precipitate after a reaction with a halide in solution, plus dilute nitric acid and silver nitrate solution.

If the precipitate is white - a chloride is present

If the precipitate is cream - a bromide is present

If the precipitate is yellow - a iodide is present