This is a group of very reactive elements. The alkali metals have a number of important applications. Lithium is the lightest metal, and usually floats on the oil in which it is stored. Aluminium-lithium alloys can be used to produce aircraft components 15% lighter than those made from other metals. These new 'super-light' alloys also have superplastic properties. Under stress, they can be stretched up to ten times without cracking. This allows them to be pressed easily into complex shapes using tools which are much simpler and cheaper than those normally required.

The use of lithium cells or 'batteries' in electronic watches and other devices may be familiar. Lithium cells produce a relatively high voltage and have a much longer useful life than other types of primary cell. Lithium cells are used in heart pacemakers.

The colour of the flame that is sometimes produced when an alkali metal reacts with water is characteristic of that metal. Street lamps that produce an orange light are sodium vapour lamps.

Group VII

The halogens have many useful compounds. Silver halides are used in photography. Photographic film is coated with an emulsion containing silver bromide. A negative image is produced when the silver salt on the film is reduced by exposure to light.

Organohalogen compounds are used as pesticides, solvents, anaesthetics and, in the case of CFCs, as refrigerants and aerosol propellants. There is concern about the environmental impact of some of these compounds. The pesticide DDT is an organochlorine compound which is decomposed very slowly and can accumulate in the food chain. Since its introduction during the Second World War, it has endangered some species of predatory birds, and it is now banned in a number of countries.

Metals

The reactions which are shown in this part of the programme could also be used to teach some of the basic concepts of redox chemistry. The reaction of a metal oxide with a more reactive metal provides a simple example by which to describe redox in terms of oxygen loss and gain. The displacement reactions involving metals and their aqueous solutions can also be interpreted in terms of electron transfer.