The metallic bond and properties of metals
Metal atoms lose their outer electrons easily. In a piece of metal, the atoms become positive ions packed in a neat pattern (a lattice), and the lost electrons move freely among them like a sea. The pull between the positive ions and the electron sea is the metallic bond.
- Good conductors of electricity and heat: free electrons carry charge and energy.
- Lustre (shine): free electrons reflect light.
- Malleable and ductile: layers of ions can slide without breaking the bond, so metals beat into sheets and draw into wires.
- High melting points and strength (most metals); mercury is a liquid, sodium and potassium are soft.
Chemical behaviour: metals lose electrons to form positive ions. They react with oxygen (basic oxides), water (very reactive ones give hydrogen and alkali), dilute acids (salt + hydrogen) and salt solutions of less reactive metals (displacement). The reactivity series lists them: K, Na, Ca, Mg, Al, Zn, Fe, Pb, (H), Cu, Ag, Au.
Ores and concentration
A mineral is a natural compound in the Earth. An ore is a mineral from which a metal can be taken out at a profit, for example haematite (Fe₂O₃), bauxite (Al₂O₃·2H₂O), zinc blende (ZnS) and copper pyrites (CuFeS₂). The unwanted sand and rock is gangue.
Concentration removes the gangue:
- Gravity (hydraulic) washing: water washes away light gangue; heavy ore stays.
- Froth flotation: for sulfide ores. Oil sticks to ore grains, air bubbles carry them up in a froth; wet gangue sinks.
- Magnetic separation: for magnetic ores like magnetite.
- Leaching: the ore is dissolved in a chemical, for example bauxite in sodium hydroxide, gold in cyanide.
Three ways to get the metal: pyro, hydro, electro
Getting a metal from its compound is reduction (the metal ion gains electrons, or the oxide loses oxygen).
1. Pyrometallurgy (heat): sulfide ores are first roasted in air: 2ZnS + 3O₂ → 2ZnO + 2SO₂. Carbonate ores are calcined (heated with little air): ZnCO₃ → ZnO + CO₂. The oxide is then reduced with carbon or carbon monoxide: ZnO + C → Zn + CO. In the blast furnace: Fe₂O₃ + 3CO → 2Fe + 3CO₂; limestone removes sand as slag, CaO + SiO₂ → CaSiO₃. Very active metals can also reduce oxides: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ (thermite, used to weld rails).
2. Hydrometallurgy (solutions): the ore is dissolved, then a more reactive metal pushes the metal out: Fe + CuSO₄ → FeSO₄ + Cu. Gold: leached with cyanide, then displaced by zinc. Good for low-grade ores.
3. Electrometallurgy (electricity): for the most reactive metals (K, Na, Ca, Mg, Al). Carbon cannot remove their oxygen. A molten compound is electrolysed: at the cathode Na⁺ + e⁻ → Na; at the anode 2Cl⁻ → Cl₂ + 2e⁻. Aluminium: alumina dissolved in molten cryolite (lowers the melting point). In water solution, hydrogen would form at the cathode instead of sodium, so a melt is needed. Less reactive metals (Cu, Zn) can be got from solutions.
Refining, the galvanic cell and corrosion
Refining makes the metal pure. In electrolytic refining of copper, impure copper is the anode, a thin pure copper sheet is the cathode, and copper sulfate is the electrolyte. Pure copper builds up on the cathode; impurities fall as anode mud (which contains silver and gold). Other methods: distillation (zinc, mercury), liquation (tin), zone refining (very pure silicon).
Galvanic cell: two different metals in salt solutions joined by a wire make electricity. The more reactive metal (for example zinc) loses electrons (anode, −) and the less reactive one (copper) gains them (cathode, +). A Zn–Cu cell gives about 1.1 V.
Corrosion is a metal slowly reacting with air, water or chemicals. Rusting of iron needs both water and oxygen: iron becomes hydrated iron(III) oxide. Protection: paint, oil or grease, plastic coating, galvanising (a zinc coat; zinc corrodes first and protects iron even if scratched — sacrificial protection), tinning, chrome plating, and alloying.
Alloys
An alloy is a mix of a metal with other metals or a non-metal, made by melting them together. Different-sized atoms stop the layers sliding easily, so alloys are usually harder and stronger; they may also resist corrosion or melt at lower temperatures.
- Steel: iron + a little carbon — strong.
- Stainless steel: iron + chromium + nickel — does not rust (cutlery, utensils).
- Brass: copper + zinc — taps, musical instruments.
- Bronze: copper + tin — statues, medals.
- Solder: lead + tin (or lead-free tin alloys) — low melting point for joining wires.
- Duralumin: aluminium + copper + magnesium — light and strong for aircraft.
- 22-carat gold: 22 parts gold + 2 parts copper or silver — harder for jewellery.
Try it at home
Rust race (with an adult): put three iron nails in three glasses: one in tap water (half covered), one in boiled cooled water with a layer of oil on top, one in a dry closed jar with a spoon of dry rice to soak up moisture. Check after a week. Which rusted most? Water + air together cause rust. In the 3D, step 6, pick Fe and Zn to compare.
Key formulas and definitions
- Roasting: 2ZnS + 3O₂ → 2ZnO + 2SO₂
- Calcination: ZnCO₃ → ZnO + CO₂
- Reduction: ZnO + C → Zn + CO; Fe₂O₃ + 3CO → 2Fe + 3CO₂
- Slag: CaO + SiO₂ → CaSiO₃
- Displacement: Fe + CuSO₄ → FeSO₄ + Cu
- Electrolysis of molten NaCl: Na⁺ + e⁻ → Na (cathode), 2Cl⁻ → Cl₂ + 2e⁻ (anode)
- Thermite: Fe₂O₃ + 2Al → 2Fe + Al₂O₃
Worked examples
1. Which method would you use for (a) sodium, (b) zinc, (c) gold?
(a) Sodium is very reactive: electrolysis of molten NaCl. (b) Zinc is in the middle: roast ZnS to ZnO, then reduce with carbon. (c) Gold is found free (native): just separate it, or leach with cyanide and displace with zinc.
2. Fe₂O₃ has a molar mass of 160 g/mol. How much iron can be made from 320 kg of pure Fe₂O₃? (Fe = 56)
1 mol Fe₂O₃ (160 g) gives 2 mol Fe (112 g). 320 kg ÷ 160 = 2 kmol Fe₂O₃ → 4 kmol Fe = 4 × 56 = 224 kg of iron.
3. Why is sodium not obtained by electrolysis of aqueous NaCl?
In water, H⁺ (from water) is reduced more easily than Na⁺, so hydrogen gas forms at the cathode. Molten NaCl has no water, so Na⁺ is reduced to sodium.
4. An ore has 5% copper by mass. How many tonnes of ore give 1 tonne of copper (no losses)?
5% means 5 t copper per 100 t ore. For 1 t: 100 ÷ 5 = 20 t of ore.
Common mistakes
- Calling every mineral an ore. An ore is a mineral from which the metal can be extracted profitably.
- Mixing up roasting and calcination: roasting heats sulfide ores in plenty of air; calcination heats carbonate ores with little or no air.
- Thinking aluminium does not corrode. It does, but a thin oxide layer forms at once and protects the metal underneath.
- Writing that the cathode is where oxidation happens. Reduction always happens at the cathode; oxidation at the anode.