Chemical Properties

Chemical Properties of Group 14

Oxidation states. With four valence electrons, Group 14 elements mainly show +4 and +2 states (carbon additionally shows negative states in its many hydrides/organic compounds). Since the sum of the first four ionisation enthalpies is very large, +4-state compounds are generally covalent, not ionic. Down the heavier members, the tendency to adopt +2 instead of +4 grows in the order $Ge < Sn < Pb$ — the inert-pair effect again, this time for the $ns^2$ pair. Overall stability pattern: C and Si are almost exclusively +4; Ge is mainly +4 with only a few +2 compounds; Sn shows both, with $Sn(II)$ acting as a reducing agent; and for Pb, +2 is the STABLE state while $Pb(IV)$ compounds are strong oxidisers.

In the +4 state the central atom has a complete octet (8 electrons, e.g. carbon in $CCl_4$) — these are "electron-precise" molecules that normally neither donate nor accept electron pairs. Carbon's covalence is capped at 4 (no d orbitals), but the heavier members, having accessible d orbitals, can expand their covalence — enabling their halides to hydrolyse and form complex ions such as $SiF_6^{2-}$, $[GeCl_6]^{2-}$, $[Sn(OH)_6]^{2-}$ (central atom $sp^3d^2$ hybridised).

Reactivity toward oxygen. All members form oxides on heating — mainly a monoxide (MO) and a dioxide ($MO_2$); SiO exists only at high temperature. Higher-oxidation-state oxides are generally more acidic: $CO_2$, $SiO_2$, $GeO_2$ are acidic; $SnO_2$ and $PbO_2$ are amphoteric. Among the monoxides, CO is neutral, GeO is acidic, while SnO and PbO are amphoteric.

Reactivity toward water. Carbon, silicon and germanium are unaffected by water. Tin decomposes steam:

Lead resists water, most likely because a protective oxide film forms on its surface.

Reactivity toward halogens. These elements form halides $MX_2$ and $MX_4$. Except carbon, every member reacts directly with halogens. Most $MX_4$ are covalent and $sp^3$-hybridised (tetrahedral); $SnF_4$ and $PbF_4$ are exceptions and are ionic. $PbI_4$ does not exist — the Pb–I bond that would initially form does not release enough energy to promote and unpair lead's $6s^2$ electrons. Dihalides ($MX_2$) become progressively more stable from Ge to Pb; for germanium $GeX_4$ is more stable than $GeX_2$, while for lead the reverse holds — $PbX_2$ is more stable than $PbX_4$. All tetrachlorides except $CCl_4$ hydrolyse readily, because the central atom can accept the lone pair from water's oxygen into an empty d orbital — carbon, lacking d orbitals, resists hydrolysis entirely. $SiCl_4$'s hydrolysis proceeds via silicon accepting water's oxygen lone pair, losing HCl to give a chlorohydroxysilane intermediate, and finally forming silicic acid, $Si(OH)_4$, after further steps. …