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A multitude of metals
A multitude of metals

There are just over one hundred elements and most of these are metals. Many versions of the periodic table of the elements include a line of demarcation between metals and the rest. Since metals have been worked for thousands of years you would expect a clear definition to be available. This is where the problems begin. Like the apocryphal visitor to the Art Gallery who knew nothing about art but knew what he liked, most of us know a metal when we see one. We all know that metals are solid but mercury is a liquid metal. Metals are dense; sodium floats on water. Metals are hard but potassium cuts like cheese. Metals conduct electricity; what about graphite, a non-metal that conducts?

 

To distinguish metals from other elements we need a combination of properties, concentration on single properties invites counter-examples. There is a continuum in the properties of elements on moving across the periodic table. Near the demarcation line are the semi-metals like germanium, silicon, gallium and arsenic. Their properties bridge those of the metals and non-metals. These four elements represent the history of the semiconductor industry, starting with the first generation transistors of germanium up to modern gallium arsenide components.

 

Some metals inspire very strong reactions. The antagonist of the fictional James Bond was appropriately named Auric Goldfinger after the Latin for gold. The pursuit of gold goes back into prehistory and gold rushes have continued right up to the present day in South America. No one who has seen the golden treasure of Pharaoh Tutankhamen or the Greek burial masks from Mycenae could doubt the aesthetic appeal of gold.

 

The lack of chemical reactivity of gold explains both why the pure metal is found as native gold and why it does not corrode or lose its surface lustre. Much ingenuity has been used to simulate real gold. The Indian goldsmiths of South America developed a technique 1500 years ago using a mixture of copper, silver and gold.  A combination of heating, treatment with acids and polishing left a surface of pure gold. The technique is called depletion gilding. The German chemist Fritz Haber (1868-1934) developed a method of extracting gold from seawater to pay war reparations after the First World War. Unfortunately he overestimated the amount of gold present in the sea and the technique was uneconomic.

 

The alchemists, including the famous Isaac Newton, devoted their energies to discovering the secrets of transmutation, changing base metals like copper or lead into gold. Alchemy has been studied for thousands of years but never succeeded. The benefit to science was the development of a whole range of chemical techniques. The ancient Greeks thought of transmutation as the death and corruption of one metal followed by the resurrection of a new one, gold.

 

Equally strong feelings are engendered by another metal, plutonium. Plutonium was discovered in 1940 and is an artificial element, none occurs naturally on Earth. It is produced from spent nuclear fuel and has two main uses, as a nuclear fuel itself and for nuclear weapons.

 

Plutonium is both dangerously radioactive and chemically very toxic, hence its generally very bad press.

 

Human history can be superficially divided into various Ages. First was the Stone Age based upon the use of flint (silica) implements. Next came the Bronze Age, starting as early as 6500BC in Anatolia. Bronze is an alloy of tin and copper whose natural ores often occur together. Tin is much rarer than copper so that, where tin was scarce, a region might move straight from the use of copper to that of iron, missing out bronze altogether. Bronze is harder than either pure metal.

 

Lastly came the Iron Age dating from about 1000BC. Looking at the distribution of metals in the Earth's crust one question arises. Since aluminium is much more common than iron, why was there no Aluminium Age? Aluminium is a very reactive metal and therefore difficult to extract. It was not until 1854 that the French chemist St Claire Deville extracted kilogram quantities of aluminium. It was then more expensive than gold. The Emperor of the French, Napoleon III (1808-1873) had special cutlery made of aluminium to impress important guests. Less favoured diners had to make do with gold cutlery. Aluminium is now so cheap, owing to the availability of cheap electricity on a large scale, that we use it for cooking foil and drinks cans. Deville used sodium metal in his original extraction process for aluminium. This same metal is used as a molten metal coolant in many nuclear power stations. Since the reaction of sodium with water is known to be violent, all water must be excluded from the cooling system. The best known metal associated with the nuclear industry is uranium. Its ores were discarded as waste in the 19th century and many old mine dumps in Devon and Cornwall are radioactive because of the uranium minerals present. The use of uranium as a nuclear fuel has made it a valuable raw material.


 
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