Group 14 — the carbon family — contains carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb). Their valence configuration is ns2np2, giving four valence electrons and a characteristic oxidation state of +4. The group shows a clear non-metal → metalloid → metal transition: carbon is a non-metal, silicon and germanium are metalloids, and tin and lead are metals.
Down the group atomic radius increases and ionisation enthalpy decreases overall, though small irregularities arise from d- and f-block contractions. The +4 state dominates the lighter elements while the +2 state becomes progressively more stable for the heavier ones, again because of the inert pair effect. As a result PbCl4 is unstable and PbCl2 is stable, while CCl4 and SiCl4 are perfectly stable. Pb4+ and Sn4+ compounds are oxidising; Sn2+ is a reducing agent.
Anomalous behaviour of carbon. Like boron, carbon stands apart from its heavier congeners owing to its small size, high electronegativity, high ionisation enthalpy and lack of valence d-orbitals. Two consequences are especially important. First, carbon's small size and the strength of the C-C bond give it an exceptional ability to bond to itself — catenation. The tendency to catenate falls sharply down the group (C >> Si > Ge ≈ Sn > Pb) because the element-element bond energy decreases. Second, carbon readily forms strong pπ-pπ multiple bonds (C=C, C≡C, C=O), so CO2 is a discrete molecule with two C=O bonds; silicon cannot form effective pπ-pπ bonds, so SiO2 is a giant covalent (network) solid.
Allotropes of carbon. In diamond each carbon is sp3 hybridised and bonded tetrahedrally to four others in a rigid three-dimensional network — making it the hardest natural substance and a non-conductor. In graphite each carbon is sp2 hybridised, forming flat hexagonal layers held together by weak van der Waals forces; the fourth, delocalised electron makes graphite a good conductor and a soft lubricant. Fullerenes (e.g. C60, buckminsterfullerene) are discrete cage molecules of sp2 carbons arranged in pentagons and hexagons like a football.
Oxides of carbon. Carbon monoxide (CO) is a neutral, colourless, highly poisonous gas (it binds haemoglobin more strongly than O2) and an excellent reducing agent in metallurgy. Carbon dioxide (CO2) is a linear, acidic gas; in excess it forms carbonic acid and it is the chief greenhouse gas. Producer gas (CO + N2) and water gas (CO + H2, also called synthesis gas) are important industrial fuels made by passing air or steam over red-hot coke.
Silicon and its compounds. Silicon is the second most abundant element in the crust. Silica (SiO2) is a hard, high-melting covalent network solid (quartz). Silicones are water-repellent polymers with the repeating (R2SiO) unit, used as sealants, lubricants and water-proofing agents. Silicates are built from SiO44- tetrahedra sharing corners; zeolites are three-dimensional aluminosilicates with cavities, used as molecular sieves, ion-exchangers (water softening) and shape-selective catalysts in petroleum cracking.