Key Si vs C differences: (1) Si has d-orbitals → can expand octet ($SiF6^{2-}$, coordination number 6); C cannot. (2) SiCl4 is hydrolysed by water (d-orbitals allow nucleophilic attack by H2O); CCl4 is inert to hydrolysis. (3) SiO2 is a giant covalent network (mp ~1700°C); CO2 is a gas (molecular, weak intermolecular forces). (4) Si-O bonds are very strong (dpi-ppi bonding), forming the basis of silicate chemistry. (5) Si doesn't form multiple bonds to itself or others. Silicones are synthetic polymers (R2SiO)n with Si-O backbone and organic groups — water-repelling, flexible, heat-resistant. Silicates have $SiO4^{4-}$ tetrahedral units sharing corners.