Metals and Non-Metals: Properties, Ores and Alloys

Of the elements known to chemistry, most are metals, a smaller group are non-metals, and a handful sit between the two as metalloids. The division is not a matter of naming but of behaviour: metals give up electrons and form positive ions, non-metals take up electrons and form negative ions, and almost every property that follows, from lustre to the nature of the oxide, can be traced to that single difference. This chapter supplies a large share of the chemistry asked in SSC, Railway and State examinations, because every fact in it is a fact that can be stated in one line.

Physical properties, and the exceptions that are asked

Metals are lustrous, malleable, ductile, sonorous, good conductors of heat and electricity, generally dense and generally high melting. Non-metals are brittle in the solid state, dull, non-sonorous and poor conductors. The exceptions matter more in an examination than the rule, because that is where the question is set. Mercury is the only metal liquid at room temperature. Sodium and potassium are so soft that they can be cut with a knife, and gallium and caesium melt in the warmth of the hand. Lithium is the lightest metal and osmium the densest. Gold is the most malleable and the most ductile metal, which is why a single gram can be drawn into a wire hundreds of metres long. Silver is the best conductor of electricity, followed by copper, and silver is also the best conductor of heat. Among non-metals, iodine is lustrous, graphite conducts electricity, and diamond, another form of carbon, is the hardest natural substance and an excellent conductor of heat.

Chemical behaviour

Metals are electropositive: they lose electrons to form cations, and their oxides are basic, turning red litmus blue when soluble. Non-metals are electronegative: they gain electrons to form anions, and their oxides are acidic, so carbon dioxide dissolves to give carbonic acid and sulphur dioxide gives sulphurous acid. Aluminium oxide and zinc oxide are amphoteric, reacting with both acids and bases, and this pair is asked directly. A metal placed higher in the reactivity series displaces a metal lower down from its salt solution. The order to memorise runs potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, then hydrogen, then copper, mercury, silver, gold and platinum. Metals above hydrogen displace it from dilute acids; those below do not, which is why copper does not react with dilute hydrochloric acid. The thermite reaction, in which aluminium powder reduces iron oxide to molten iron, is a displacement reaction used to weld railway tracks.

Important ores

Extraction follows a fixed sequence. The ore is first concentrated, by froth flotation for sulphide ores and by magnetic separation where one component is magnetic. A carbonate ore is then calcined, that is heated in the absence of air, and a sulphide ore is roasted, heated in air, and both become the oxide. The oxide is reduced to the metal, by carbon for moderately reactive metals and by electrolysis for the most reactive, which is why aluminium is obtained from molten bauxite by electrolysis and never by carbon reduction. The crude metal is refined, usually by electrolytic refining, where the impure metal is the anode and the pure metal is deposited at the cathode.

Alloys

• Brass is copper and zinc; bronze is copper and tin. These two are the most frequently asked pair in the whole chapter.
• Solder is lead and tin, used for joining electrical wires because of its low melting point.
• Stainless steel is iron with chromium and nickel, which is why it resists rusting.
• Duralumin is aluminium with copper, magnesium and manganese, and is used in aircraft for its strength with low weight.
• German silver is copper, zinc and nickel, and contains no silver at all.
• Bell metal and gun metal are both based on copper and tin, gun metal with zinc added.
• Nichrome, of nickel and chromium, has high resistance and is used in the heating elements of appliances.
• An amalgam is any alloy in which one of the metals is mercury.

Corrosion and its prevention

Corrosion is the slow eating away of a metal by the air and moisture around it. Iron rusts to hydrated iron oxide, and both oxygen and water must be present for rusting to begin; in dry air or in water free of dissolved air, iron does not rust. Copper develops a green coating of basic copper carbonate and silver a black coating of silver sulphide. Corrosion is prevented by painting, oiling and greasing, by electroplating, by alloying as in stainless steel, and by galvanisation, in which iron is given a coating of zinc; the zinc protects the iron even where the coating is scratched, because zinc is more reactive than iron. Aluminium is protected by its own thin oxide layer, thickened deliberately in the process called anodising. Sodium and potassium are stored under kerosene because they react violently with air and water, while white phosphorus is stored under water because it catches fire in air.

Metalloids and a note on uses

Boron, silicon, germanium, arsenic, antimony and tellurium show properties of both classes and are called metalloids; silicon and germanium are the basis of semiconductor devices. Among the uses examiners ask for: copper and aluminium for electrical wiring, aluminium foil for packaging, tungsten for lamp filaments because of its very high melting point, mercury in thermometers, and aqua regia, a mixture of three parts concentrated hydrochloric acid to one part concentrated nitric acid, which dissolves gold and platinum.