Metals, Non-metals, Alloys & Metallurgy

Around us, about three out of every four known elements are metals, and the rest are non-metals and a handful of in-between metalloids. The sorting is not about looks. It depends on one thing: how an atom behaves with its outer electrons. A metal atom has few outer electrons and loses them easily to become a positive ion (cation), which is why metals are called electropositive. A non-metal atom is close to a full outer shell, so it gains or shares electrons. Once you hold on to this single idea, every property in the table below follows from it.

Why do metals conduct and shine? In a metal the outer electrons are not tied to any one atom. They form a free 'sea' of electrons that moves through the whole piece. When a voltage is applied, the sea drifts and we get an electric current; when heat arrives, these electrons carry it quickly; when light falls on them, they bounce it back, and that is the shine. When you hammer a metal, the layers of atoms slide over one another but the electron sea keeps holding them together, so the metal bends instead of breaking. In a non-metal the electrons are tightly held, so none of this happens.

Amphoteric oxides and metalloids. Some metal oxides show both faces: they react with acids as bases and with strong alkalis as acids. These are amphoteric oxides: Al₂O₃, ZnO, PbO, SnO, BeO. For example, Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O and Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O. Metalloids sit on the staircase between metals and non-metals and carry properties of both: boron, silicon, germanium, arsenic, antimony, tellurium. Silicon and germanium are semiconductors, so they sit inside chips and transistors.

Not all metals are equally eager to react. Sodium bursts into flame in the open and has to be kept in kerosene, iron slowly rusts, and gold stays shiny in a grave for centuries. Chemists lined metals up in order of how eagerly they lose electrons. This list is the reactivity series (activity series). Think of it as a ladder: the higher a metal stands, the more reactive it is, the more easily it loses electrons, the stronger a reducing agent it is, and the harder it is to extract from its ore.

Only a few metals, such as gold and platinum, are found free in nature, because they are very unreactive. Most metals are found as compounds (oxides, sulphides, carbonates, chlorides) mixed in rocks and soil. The natural material in which a metal is found is a mineral. A mineral from which the metal can be extracted profitably is an ore. So every ore is a mineral, but not every mineral is an ore: clay contains aluminium, yet the metal is extracted from bauxite. The earthy impurities (sand, clay, rocky matter) attached to the ore are called gangue (matrix). The whole science of extracting a pure metal from its ore is metallurgy.

Why does the method depend on the position in the series? The higher a metal is, the tighter it clings to oxygen, so only the strongest 'push' (electricity) can pull it out: K, Na, Ca, Mg, Al are extracted by electrolysis of their molten compounds. Metals in the middle (Zn, Fe, Pb, Sn and others) cling less and are reduced by carbon after the ore has been converted to oxide. Metals at the bottom hardly hold on to oxygen at all: mercury is obtained just by heating its sulphide (cinnabar) in air, and silver and gold occur free or are won by simple methods.

Leave an iron nail in a damp place and in a few weeks it is covered with a reddish-brown flaky coating. Leave a silver spoon in the open and it turns dull black. A copper pot turns green. All these are corrosion: the slow eating away of a metal by the air, water or chemicals around it, when the metal changes back to a more stable compound. India loses a large amount of iron to rust every year, so knowing how to stop it is practical science.

Why is galvanisation better than tinning? Both put a metal coat on iron. If the coat is scratched, air and water reach the iron. Zinc is more reactive than iron, so it gives up electrons first and corrodes in place of iron: iron stays safe even when the coating is damaged. Tin is less reactive than iron, so once the coat is broken the iron becomes the sacrificial one and rusts even faster than before. This 'who is more reactive' idea is exactly the idea behind magnesium blocks on ship hulls.

A pure metal is often too soft, too easily corroded or too expensive for real use. Pure iron is soft, pure gold is too soft for jewellery, and pure copper cannot make a good bell. So metallurgists mix a metal with one or more other elements, metals or even non-metals like carbon, in a molten state, and let the mixture cool. The result is an alloy: a homogeneous mixture (not a compound) of a metal with other elements. Because an alloy is a mixture, its components are not in a fixed chemical ratio and they keep their own properties, but the blend is harder, stronger, more resistant to corrosion, or with a lower melting point than the pure metal.

Why does alloying make a metal stronger? In a pure metal all atoms are of the same size and lie in neat layers, which can slide over each other easily, so the metal is soft. In an alloy, atoms of a different size are scattered among them. They jam the layers and stop them from sliding. That is why brass is harder than copper and steel is harder than iron. Alloying also lowers the melting point (solder melts easily, which is exactly why it is used for joining), and changes electrical resistance (nichrome has high resistance and heats up).

Look around your room and count the metals. The wires in the walls are copper or aluminium, the fan is aluminium and steel, the tube-light has mercury vapour, the bulb filament is tungsten, the phone battery contains lithium, and even the screen has traces of silver and gold. Each metal is chosen for a reason: its conductivity, lightness, strength, resistance to corrosion or price. This topic is the 'who is used for what' list that the exams love.

Useful compounds of metals. Blue vitriol is copper sulphate pentahydrate, CuSO₄·5H₂O: it is used as a fungicide (Bordeaux mixture with lime) and in electroplating. Green vitriol is FeSO₄·7H₂O (water purification, tonics); white vitriol is ZnSO₄·7H₂O. Ruby is aluminium oxide (Al₂O₃) coloured red by traces of chromium; sapphire is also Al₂O₃ coloured blue by iron and titanium. Mica is a silicate mineral, an excellent electrical insulator that is also heat-resistant, so it is used in electrical industries (insulators, capacitors, heating appliances).

Metals are the gifts of the earth, but life itself is built from non-metals. Over 95 per cent of your body weight is oxygen, carbon, hydrogen and nitrogen, and the air you breathe is almost entirely nitrogen and oxygen. Non-metals have 4 to 8 electrons in their outer shell, so they take or share electrons. Here we meet the non-metals that exams ask about most: hydrogen, nitrogen, oxygen, phosphorus, sulphur, the halogens and the noble gases. (Carbon, the most special non-metal of all, has a chapter of its own.)

Why are noble gases noble? Their outer shell is already full (two electrons for helium, eight for the rest). An atom with a full shell has no urge to gain, lose or share electrons, so it stays alone, as a single atom (monatomic). That is also why they have very low boiling points: there are hardly any forces holding the atoms together. Because they are so unreactive, they are used where a safe, inert atmosphere is needed: argon around a welding arc, helium in a balloon, neon in a glowing sign. (Xenon and krypton can be forced to react with fluorine or oxygen, but only under special conditions.)