Chapter 4: The World of Metals and Non-metals

Malleability is the property of a material that lets it be hammered or rolled into thin sheets without breaking. Most metals are malleable. Iron, copper, aluminium, gold and silver can be beaten flat. Non-metals such as coal, sulfur and wood are not malleable; they break into pieces or crumble when hit. Materials that shatter when hammered are called brittle. Thin sheets of aluminium used to wrap food and thin silver or gold foils on sweets show how malleable metals can be.

Ductility is the property of a metal that lets it be drawn into thin wires without breaking. Most metals are ductile. Copper, aluminium, iron and gold can be drawn into wires. A very small amount of gold can be drawn into a wire several metres long. Non-metals such as sulfur and coal are not ductile; they snap or crumble when pulled. Ductility is why electrical wires, jewellery chains, and springs can be made from metals.

Lustre is the shine on the surface of a material. Metals usually have a shiny surface when they are freshly cut or polished. Non-metals such as coal, wood, and sulfur are generally dull. Sonority is the property of making a ringing sound when struck. Metals are sonorous: a school bell, a brass vessel, and a steel plate ring out when hit. Wood, plastic, and coal give a dull thud. These properties help to identify metals but must be used together with other tests.

Conduction of heat is the transfer of heat through a material from the hot part to the cooler part. Metals are good conductors of heat. When one end of a metal rod is heated, the other end soon becomes hot. Materials that do not allow heat to flow easily are poor conductors of heat or heat insulators. Wood, plastic and air are poor conductors. This is why cooking vessels are made of metals with handles of wood or plastic.

Metals are good conductors of electricity. This is why wires used in homes, factories and gadgets are made of metals such as copper and aluminium. Materials that do not conduct electricity are insulators; plastic, rubber, glass and dry wood are examples. To test a material, we place it in an electric circuit with a cell and a bulb. If the bulb glows, the material is a conductor. If it does not glow, it is an insulator. Most non-metals do not conduct electricity; however, graphite is a non-metal that does.

Many metals react slowly with the oxygen and moisture in air and lose their shine. When iron stays in damp air, it forms a reddish-brown coating called rust; this is rusting. Rusting needs both oxygen and water. Copper becomes dull and then green with time. Silver turns black from tarnishing. Aluminium forms a thin protective layer that stops further damage. Gold and platinum hardly react at all. The slow damage of metals by air and moisture is called corrosion. We can prevent rust by painting, oiling, or coating iron with zinc (galvanising).

Non-metals are materials that usually lack the properties of metals. Solid non-metals like coal (carbon) and sulfur are generally dull, brittle, and not sonorous. They are usually poor conductors of heat and electricity. Many non-metals are gases at room temperature, such as oxygen and nitrogen. Bromine is a non-metal that is liquid at room temperature. Non-metals are essential: oxygen is needed for breathing, carbon forms the basis of living things, and sulfur is used in matches and medicines.

A few substances do not fit the general pattern. Mercury is a metal that is liquid at room temperature. Sodium and potassium are soft metals that can be cut with a knife, and they are stored under kerosene because they react quickly with air and water. Graphite is a non-metal that conducts electricity. Iodine is a non-metal that is shiny. Diamond is a non-metal that is the hardest known natural substance. These exceptions remind us to test several properties rather than rely on only one.