The carbon group. Carbon

The carbon group (main subgroup of group IV of the periodic table) consists of C, Si, Ge, Sn, Pb; their outer shell is ns²np² and they are p-elements. In compounds they are usually bivalent or tetravalent; the highest oxidation state is +4 (CO₂, SiO₂) and the lowest is –4 (CH₄, Mg₂Si). Carbon also shows +2 (CO). Down the group the atomic radius grows and metallic character increases: C and Si are nonmetals, Sn and Pb are metals. Carbon occurs free (diamond, graphite), in carbonates (CaCO₃, MgCO₃, dolomite CaCO₃·MgCO₃), in coal, oil, natural gas and in all living organisms. In diamond every atom forms nonpolar covalent bonds with four others in a tetrahedron — it is the hardest substance and does not conduct; graphite is layered with weak bonds between layers, so it is soft, metallic-looking and conducts current. Carbyne (linear chains) and fullerene (closed carbon “balls”) are further allotropes. Plants take up CO₂ in photosynthesis, while respiration, decay, burning and volcanoes return CO₂ to the atmosphere.