Account for the following observations about the elements in Group 14 (C - Pb) and their compounds i โ Descriptive Chemistry / Bonding Chemistry Question
Problem Context
Account for the following observations about the elements in Group 14 (C - Pb) and their compounds in terms of bonding principles.
Carbon (in the form of diamond) and silicon have the same tetrahedral structure but very pure silicon is a semiconductor while diamond is an electrical insulator.
Model Answer
Electrons are held in covalent bonds between C atoms in diamond. Electrons in Si (with a lower IE) can be excited to move through lattice. Valence band (fully occupied) and conduction band (empty) are widely separated in energy. In Si the energy gap between the occupied valence band and the empty conduction band is small enough that some electrons are excited at room temperature.
Carbon and silicon both form tetrachlorides, but while CCl4 does not react with H2O at 25 หC, SiCl4 reacts violently with H2O at 25 หC.
Model Answer
The carbon atom in CCl4 is protected from H2O attack by the Cl atoms. C has no orbitals available to bond with H2O. Si is larger, allowing H2O access and has d orbitals available to bond with H2O molecule in addition to having a more polar bond (Si-Cl vs. C-Cl)
Carbon forms compounds containing chains of carbon atoms but the tendency of the elements in the family to bond to one another in this fashion decreases with increasing atomic number.
Model Answer
C-C bonds are very strong because of the substantial overlap of the orbitals. Bond strength decreases down the family because the larger orbitals do not overlap as efficiently leading to weaker bonds.
Germanium, tin, and lead form stable chlorides in which they exhibit oxidation states of +2 and +4 but the +4 state decreases in stability relative to the +2 state with increasing atomic number.
Model Answer
+2 and +4 oxidation states result from the involvement of electrons from p or s+p orbitals, respectively. Electrons in s orbitals become less available for bonding as atoms become larger due to penetration of s orbitals and their greater attraction to higher charged nuclei.