More than 80 % of the elements in the periodic table are metals. Their shared physical properties — metallic lustre, electrical and thermal conductivity, malleability and ductility — have one cause: metallic bonding. Chemically, metals are always reducing agents: in reactions they lose electrons and become positive ions (cations). Metals have marked whole eras of human history (the Copper, Bronze and Iron Ages), and today construction, transport, electrical engineering and electronics would be impossible without them. This topic covers the general properties of metals, how they are extracted and how they are protected from corrosion; particular groups are treated in detail in "Alkali metals" and "d-block metals".
§1. Position in the periodic table and structure
Metals lie in the left and lower part of the periodic table — below and to the left of the boron–astatine diagonal. They include all s-block elements (except H and He), the d- and f-block elements and some p-block elements (Al, Sn, Pb, Bi and others).
Metal atoms usually have 1–3 outer electrons, a large radius, and low ionisation energy and electronegativity, so they lose electrons easily.
Metal cations (and atoms) occupy the lattice points, and shared delocalised electrons (the "electron sea") move between them — this is metallic bonding.
Metallic bonding is non-directional, so layers of atoms can slide over each other without breaking the bonding — metals can be hammered and drawn into wires. The free electrons carry electricity and heat and reflect light (lustre).
§2. Physical properties
The best electrical conductors: Ag > Cu > Au > Al; that is why wires are made of copper and aluminium. The conductivity of metals falls as temperature rises.
The least dense metal is Li, the densest Os; the highest-melting is tungsten (about 3400 °C, used for lamp filaments); mercury is liquid at room temperature (it freezes at −39 °C); chromium is the hardest metal and the alkali metals the softest.
Colour: most are silvery grey, gold is yellow, copper reddish.
In engineering, ferrous metals (iron and its alloys) are distinguished from non-ferrous ones; by density there are light metals (ρ < 5 g/cm³: Li, Na, Mg, Al, Ti) and heavy ones (Fe, Cu, Pb, Hg, Au). The precious metals — Au, Ag, Pt — are highly corrosion-resistant.
§3. Chemical properties
Me⁰ − ne⁻ → Meⁿ⁺ — free metals are reducing agents only
Reactivity follows the electrochemical series: Li K Ba Ca Na Mg Al Mn Zn Cr Fe Ni Sn Pb (H) Cu Hg Ag Pt Au.
With non-metals: 2Na + Cl₂ → 2NaCl, Fe + S → FeS, 2Mg + O₂ → 2MgO, 3Ca + N₂ → Ca₃N₂.
With water: Li–Na, Ca and Ba give an alkali and H₂ in the cold (2K + 2H₂O → 2KOH + H₂); Mg reacts with hot water; Zn and Fe react with steam to give an oxide and H₂ (3Fe + 4H₂O → Fe₃O₄ + 4H₂); metals after hydrogen do not react with water.
With acids: metals before hydrogen displace H₂ from dilute HCl and H₂SO₄; with HNO₃ and conc. H₂SO₄, compounds of nitrogen or sulfur form instead of H₂.
With salt solutions: a more reactive metal displaces a less reactive one from its salt (except the very reactive metals, which react with the water first).
Amphoteric metals (Al, Zn, Be) also release hydrogen with alkali solutions.
Worked example
A copper plate was placed in AgNO₃ solution and its mass increased by 15.2 g. What mass of silver was deposited?
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag.
1 mol Cu dissolves (−64 g) and 2 mol Ag deposit (+216 g): Δm = +152 g.
15.2/152 = 0.1 mol Cu → 0.2 mol Ag = 0.2·108 = 21.6 g.
Worked example
Dissolving 2.4 g of a divalent metal in excess hydrochloric acid released 2.24 L of hydrogen (STP). Identify the metal.
Only unreactive metals (Au, Pt, partly Ag and Cu) occur free in nature; the rest are found in ores as oxides, sulfides, carbonates and chlorides. Extraction means reducing the metal ions: Meⁿ⁺ + ne⁻ → Me⁰.
Pyrometallurgy — reduction at high temperature: by carbon or CO (Fe₂O₃ + 3CO → 2Fe + 3CO₂, ZnO + C → Zn + CO), by hydrogen (WO₃ + 3H₂ → W + 3H₂O) or by a more reactive metal — the thermite process (Cr₂O₃ + 2Al → 2Cr + Al₂O₃).
Hydrometallurgy — dissolving the ore and displacing the metal from solution with a more reactive metal: CuSO₄ + Fe → FeSO₄ + Cu; gold is recovered from cyanide solutions.
Electrometallurgy — the most reactive metals (Na, K, Ca, Mg, Al) are produced by electrolysis of melts; copper and zinc are refined by electrolysis of solutions.
Sulfide ores are first roasted in air to oxides: 2ZnS + 3O₂ → 2ZnO + 2SO₂, and then reduced. In Uzbekistan, the Almalyk mining and metallurgical complex produces copper, zinc and lead, and Navoiy produces gold.
§5. Corrosion and alloys
Corrosion is the spontaneous destruction of metals by their environment: chemical (in dry gases, at high temperature) and electrochemical (in the presence of an electrolyte, through galvanic couples).
Rusting of iron: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃
Protection: paint, varnish, grease; metal coatings (zinc — galvanising, tin — tinplate, chromium, nickel); sacrificial protection (attaching a more reactive metal — zinc or magnesium — which corrodes instead); inhibitors; stainless steel (with added Cr and Ni). Alloys are mixtures of a metal with other metals or non-metals made by melting them together; they are harder and stronger than the pure metals: steel and cast iron (Fe + C), bronze (Cu + Sn), brass (Cu + Zn), duralumin (Al + Cu + Mg).
Worked example
Galvanised iron and tinplate are both scratched. Which rusts faster?
Zinc is more reactive than iron → in the galvanic couple zinc is the anode and dissolves, protecting the iron.
Tin is less reactive than iron → iron becomes the anode and corrodes quickly.
Answer: the tinplate.
Check yourself
Which metal reacts with cold water: Fe, Ca, Cu, Zn?
5.4 g of Al dissolved in excess HCl. What volume of H₂ (STP) is released?
Which pair reacts: a) Cu + FeSO₄; b) Fe + CuSO₄; c) Ag + HCl?
In a thermite reaction 30.4 g of Cr₂O₃ was reduced. What mass of Al is needed?
Which metal is attached to a buried iron pipe to stop it rusting?
1.2 g of Mg was burnt in air. What is the mass of the product?
Which metal is the best conductor of electricity?
How is aluminium produced: pyrometallurgy, hydrometallurgy or electrolysis?
Common mistakes
Writing that Na or K displaces copper from CuSO₄ solution: they react with the water.
Counting only the deposited metal when a plate’s mass changes: some metal also dissolves.
Saying free metals can act as oxidising agents.
Calling a tin coating sacrificial protection: tin is less reactive than iron.
Thinking metals conduct better when heated: their conductivity falls.