|Lecture Notes::Lec 20_27 June
© R. Paselk 2006
The Representative Elements, cont.
Group 4 Chemistry, cont.
- Silicon. Crystallizes as the pure element in a diamond lattice, forming a hard, brittle, high-melting solid with a metallic luster, but low conductivity. It is THE base of the modern semi-conductor industry.
- Silicon is also the basis of most mineralogy - in a sense the carbon analogue for the inorganic world.
- Chemistry is dominated by the chemistry of the +4 oxidation state.
- Silicon can form binary compounds with hydrogen (Silanes, SiH4, Si2H6, etc.) and halogens (SiCl4) but these compounds tend to react with oxygen to give silicon dioxide, SiO2.
- Most silicon compounds are based on silicon dioxide and silicates.
- Unlike carbon, silicon does not form double bonds with oxygen, rather it forms compounds with sp3 hybridization and four oxygens arranged tetrahedrally around the silicon.
- The simplest is SiO4-, orthosilicate.
- The acid of orthosilicate is only stable in very dilute solution, polymerizing as it concentrates.
- The orthosilicate ion is found in a few minerals such as: olivine ( Mg4SiO4) and zircon (ZrSiO4)
- Extended silicate ions are called metasilicates. Note that in each case the silicon is again bonded tetrahedrally to four oxygens.
- The simplest metasilicate is the "infinite" linear chain, SiO32-.
- Minerals based on linear metasilicate are known as pyroxenes, and include jadeite, NaAl(SiO3)2. The metasilicate chains are held together adjacently by the cations bridging the charged oxygens in these minerals.
- The next silicate has two chains linked via bridging oxygens to give a band structure (double chain). In this case the empirical formula is the Si4O116- ion.
- This structure occurs in a variety of minerals known as amphiboles, including the various asbestos minerals.
- Silicate can also form sheets with the empirical formula Si2O52-. In this case each silicon atom is crosslinked to three others via oxygen bridges, with a single free oxygen.
- When these sheets are cross bridged to each other via cations they form a variety of sheet structured minerals such as talc [Mg3(Si2O5)2(OH)2], micas, and clays. These minerals are critical as a reservoir holding cations in soils essential to plant growth.
- Silica can also form cyclic ions with the oxygens bridging the silicons, leaving two free oxygens per silicon (thus each silicon is in the center of a tetrahedron, and has two negative charges, one each on the non-bridging oxygens), such as
- cyclic-trimetasilicate (Si3O96-, a six-membered ring structure), which occurs in benitoite (BaTiSi3O9).
- cyclic-hexametasilicate (Si6O1812-, a twelve-membered ring structure), which occurs in beryl (Be3Al2Si6O18).
- Quartz has the empirical formula of SiO2.
- Quartz glass is made up of 3-D clusters of SiO2.
- Other glasses consist of random arrays of these clusters with other components to ease workability etc. such as soda Na2O, CaO, etc.
- Finally, silica chains which have been, for example, methylated on the free oxygens form a valuable series of synthetic compounds, the silicones (e.g. silicone rubber, silicon wax, silicon oils) which are more resistant to breakdown than carbon based oils and waxes.
- Germanium Like silicon it crystallizes as the pure element in a diamond lattice, forming a brittle solid. It is an important semi-conductor.
- The chemistry of germanium is much like silicon's, but it can also form octahedral compounds with six oxygens around germanium due to its larger size.
- Tin Fairly rare metal occurring as the ore cassiterite, SnO2, from which the metal is obtained by reduction with carbon: SnO2(s) + 2C(s) Sn(l) + 2CO(g). It is used in various alloys (pewter, bronze and solders) and protective plating of "tin cans" etc.
- Three allotropes:
- alpha-tin (gray tin): non-metallic, stable below 13°C, atoms bonded in diamond lattice. "Tin disease."
- beta-tin (white tin): the common, metallic form, stable from 13°C - 161°C.
- gamma-tin (rhombic tin): atoms are bonded in an orthorhombic lattice, brittle, stable above 161°C to the melting point of 232°C.
- There are two oxidation states, +4 (stannic) and +2 (stannous). The second common oxidation state of +2 demonstrate the inert pair effect.
- Note (and know) chemistry we see in lab.
- Lead Obtained from the mineral galena (PbS, often occurs in shiny, silvery, cubic crystals). May be obtained from ore by roasting in oxygen to give lead(II) oxide (PbO) and lead(II) sulfate which can then be mixed with additional PbS without air to give lead metal. It is important in alloys and solders, and particularly in lead acid batteries. It used to be used in vast quantities in leaded gasoline. Important from ancient times.
- Like tin, lead exhibits two oxidation states, +4 (plumbic) and +2 (plumbous). The second common oxidation state of +2 demonstrates the inert pair effect.
- Like most heavy metals lead is toxic ("painter's holiday"; Romans - pipes, "sugar of lead").
© R A Paselk
Last modified 27 June 2006