Carbon has the highest catenation tendency due to strong C-C bonds $\frac{348 kJ}{mol}$. Compare: Si-Si = 226, Ge-Ge = 188, Sn-Sn = 151 kJ/mol. Carbon forms ppi-ppi multiple bonds (C=C, C≡C); Si cannot (3p orbitals too diffuse for effective lateral overlap). Allotropes: Diamond (sp3, 3D tetrahedral network, hardest, insulator), Graphite (sp2, layered, delocalised pi electrons, conducts electricity, lubricant), Fullerene C60 (sp2, soccer ball cage, 12 pentagons + 20 hexagons), Graphene (single layer graphite, exceptional conductor). CO is isoelectronic with N2 (triple bond, very stable, toxic). CO2 is linear (sp, O=C=O). SiO2 is a 3D network (Si-O single bonds, very high m.p.) — contrast with discrete CO2 molecules.