Chem 3 - Development of the Periodic Table - The Halogens

The Halogens are a group of poisonous non-metals which all have coloured vapours. They are fairly typical non-metals:

- They have low melting points and boiling points

- They are also poor conductors of heat and electricity

The Halogens all look different. At room temperature fluorine is a very reactive, poisonous, pale yellow gas, while chlorine is a reactive, poisonous dense green gas.

It is important to be able to detect chlorine if it is given off. It has a very distinctive smell - you'll recognise it from swimming pools. But it is much safer to hold a piece of damp litmus paper in an unknown gas. If the damp litmus paper is beached, the gas is chlorine.

Bromine is a dense, poisonous dark orange-brown liquid which vapourises eaily - it is volatile. Iodine is a poisonous dark grey crystalline solid which produces violet-coloured vapour when we heat it.

As elements, the Halogens all exist as molecules made up of pairs of atoms, joined together by covalent bonds. (We call this type of molecule a diatomic molecule)

The way the Halogens react with other elements and compounds is a direct result of their electronic structure. They all have a highest energy level containing 7 electrons. So they just need one more to achieve a stable arrangement.

This means that the Halogens take part in both ionic and covalent bonding. It also explains why the Halogens get less reactive as we go down the group. Thats because the outer electrons get further away and are more and more shielded from the nucleus.

The Halogens all react with metals. They gain a single electron to give them a stable arrangement of electrons, forming ions with a 1- charge.

In these reactions, ionic salts (which we call metal halides) are formed. Some examples of these are Sodium Chloride, Iron Bromide and Magnesium Iodide. When we react the Halogens with other non-metals, both sets of atoms share electrons to gain a stable electronic structure. Therefore their compounds with non-metals contain covalent bonds.Examples of these bonds are Hydrogen Chloride and Tetrachloromethane.

We can use a more reactive Halogen to displace a less reactive Halogen from solutions of its salts. Bromine displaces Iodine from the solution because it is more reactive than Iodine, while Chlorine will displace both Iodine and Bromine. For example, Chlorine will displace Bromine if we bubble the gas through as solution of Potassium Bromide:

Cl2 + 2KBr -> 2KCl + Br2

Obviously Fluorine, the most reactive of the Halogens, would displace all of the others. However it reacts so strongly with water that we cannot carry out any displacement reactions in aqueous solutions.