Group 14 Elements: The Carbon Family — General Trends

Group 14, the carbon family, comprises carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb), sharing the general outer-shell configuration $ns^2np^2$. This group shows an unusually clean and complete transition in character from top to bottom: carbon is a strict nonmetal, silicon and germanium are metalloids, and tin and lead are genuine metals — making Group 14 one of the best groups in the periodic table for illustrating the general increase in metallic character on descending a group.

Like Group 13, Group 14 shows two characteristic oxidation states separated by two units: the group oxidation state $+4$ (using all four valence electrons in bonding) and the inert-pair state $+2$ (using only the two $np$ electrons, with the $ns^2$ pair remaining as a non-bonding "inert" lone pair). For carbon and silicon, the $+4$ state is overwhelmingly the more stable and more commonly encountered — carbon's entire organic chemistry, and silicon's entire silicate/silica chemistry, are built almost exclusively around the $+4$ state, with $+2$ species (like $\text{CO}$, formally $\text{C}^{2+}$-like in a simplified oxidation-state sense) being comparatively reactive and less common. Moving down the group, the inert pair effect strengthens, so that by tin, both $+2$ and $+4$ compounds are reasonably common and comparably stable, and by lead, the $+2$ state has become the more stable of the two — $\text{PbO}$ (lead(II) oxide) is a common, stable compound, while $\text{PbO}_2$ (lead(IV) oxide) is in fact a strong oxidising agent that tends to be reduced back to the $+2$ state, exactly mirroring the $\text{Tl}^{3+}/\text{Tl}^{+}$ behaviour seen in Group 13.

A second defining feature of Group 14 is the trend in catenation — the tendency of an element's atoms to bond to each other in chains, rings or networks. Carbon shows by far the strongest catenation of any element in the periodic table, forming exceptionally strong C–C single, double and triple bonds; this is the structural basis of the entire discipline of organic chemistry. Catenation ability falls away sharply on descending the group — silicon can form Si–Si chains of only modest length, germanium less so, and tin and lead show essentially negligible catenation — because the element–element bond enthalpy decreases markedly as atomic size increases down the group and the bonding electrons become more diffuse and less effectively shared. This catenation trend, and the associated structural chemistry it produces (most vividly, the several allotropes of carbon), are examined in detail in the next section. …