General chemistry of group 14 elements

General chemistry of group 14 elements

Occurrence, extraction and use of group 14 elements

Silicon is, after oxygen, the second most abundant element in the earth’s crust. It occurs in a range of minerals and sand (SiO₂, quartz). In contrast, germanium, tin and lead are relatively rare elements, with tin and lead being extracted for thousands of years from their ores. The main source for tin is cassiterite (SnO₂) and for lead galena (PbS). Germanium was first isolated from the mineral argyrodite when it was discovered, but there are no major deposits of this mineral. Germanium is nowadays mainly sourced from zinc and copper ores.

Silicon can be extracted from silicates or sand by reducing SiO₂ with coke at high temperatures at around 3000 ^∘C.

SiO₂ + 2C → Si + 2CO                          (5.1)

Germanium is extracted from zinc ores in a very complicated process as it has aqueous properties simi-lar to those of zinc. Once the germanium/zinc mixture has been sufficiently enriched with germanium, it is heated in HCl with Cl₂ in order to allow the formation of germanium tetrachloride (GeCl₄). GeCl₄ can be easily separated from ZnCl₂ because the former has a significantly lower boiling point, and after hydrolysis germanium dioxide (GeO₂) is obtained. GeO₂ can be reduced to elemental Ge in a stream of hydrogen gas. Elemental Sn is extracted from the ore cassiterite (SnO₂) via reduction with carbon. Lead is obtained from its sulfide (PbS, galena), which is first roasted in the presence of oxygen and then reduced with carbon to give elemental Pb.

PbS + 1¹∕₂O₂ → PbO + SO₂

PbO + C → Pb + CO

Silicon is used in a wide variety of applications. In nature, silicon does not exist as the pure metal and most commonly occurs in silica (including sand) and silicates. Silicon dioxide, also known as silica, is a hard substance with a high melting temperature and clearly very different from carbon dioxide. Molten silica can be used to make glass, an extremely useful material, which is resistant to attack by most chemicals except fluorine, hydrofluoric acid and strong alkalis. Silicon atoms can also be found in the class of compounds called silicones. Silicones are inert synthetic polymers with a wide range of uses including as sealants, cookware, adhesives and medical applications. Silicones contain next to silicon atoms also carbon, hydrogen and oxygen atoms (Figure 5.2).

Pure silicon metal is used in semiconductors, the basis of all electronic devices, and is most well known for its application in solar panels and computer chips. Germanium can be mainly found not only in electrical components and in semiconductors but also in some optical applications and some specialised alloys.

A semiconductor is a crystalline solid that has an electric conductivity between that of a conductor (e.g. copper) and an insulator (e.g. glass). This effect can be a result of impurities or temperature.

Metallic tin has many applications, but the most well known one is the use as lining for drink and food cans. It is also used in alloys – bronze is an alloy of copper and tin – and organotin compounds find their application in poly(vinyl chloride) (PVC) plastics. Tin is often referred to as a poor metal and it has two allotropes: grey tin and white tin.

Lead is a grey metal and most lead is used in batteries. Other major uses, such as in plumbing or as antiknock agent in petrol (tetraethyl lead, Pb(C₂H₅)₄), have declined over recent years because of the high toxicity of lead. Pb is a neurotoxin when ingested and many lead compounds are water soluble. Therefore, water lines have been replaced by specialised plastic material, and in most industrialised countries only unleaded petrol is sold.

Oxidation states and ionisation energies

Group 14 elements can have the oxidation state +2 or +4, with the latter one being the dominant one. The stability of the oxidation state +2 increases for elements further down the group, such as Sn and Pb. Group 14 elements have four valence electrons, two in the s orbital and two in the p orbital. Therefore, the first four ionisation energies increase evenly, whereas there is a significant increase to the fifth (Table 5.1).

Typical compounds of group 14 elements

The chemistry of group 14 elements is different from that of its naming element, carbon. The best example is the ability of carbon to form double and triple bonds. None of the remaining group 14 elements shows the same behaviour and there are only very few compounds known that contain silicon–silicon double and triple bonds. Carbon forms double and triple bonds through the overlap of its 2p orbitals. In contrast, the overlap of the 3p orbitals in silicon is weaker, and as a result silicon does not form multiple bonds as readily as carbon.

1. Oxides

Silicon oxides can be very complex structures, which is in contrast to the simple linear molecule carbon dioxide with its C==O bonds. The most basic molecule is SiO₂ (silica), which is an extended array of SiO₄ units where the oxygen atoms bridge the neighbouring silicon atoms. Silicon forms an array of oxides that occur in many minerals. All those are made up of SiO₄ tetrahedra forming rings or chains with an overall negative charge. 

The negative charges are balanced by metal ions (e.g. alkali and alkaline earth metals). One of the best known examples is the commercially available synthetic zeolite, which is an aluminosilicate incorporating aluminium ions. Zeolite is used as an absorbent and in the production of laundry products (Figure 5.3).

Germanium dioxide has the chemical formula GeO₂ and forms as a passivation layer on pure germanium when the semi-metal is in contact with atmospheric oxygen. GeO₂ forms structures that are similar to SiO₂ depending on the reaction conditions. Germanium monoxide (GeO) is formed when GeO₂ is heated with powdered germanium at 1000^∘C. GeO₂ is used in optical devices such as lenses and optical fibres, amongst other applications.

2. Hydrides

Silicon hydrides are called silanes and are structurally similar to their carbon analogues, with the general formula Si_nH_2n+2. Silanes are typically colourless and can be gases or volatile oils. Germanium hydrides are accordingly called germanes and these are generally less reactive than silanes. A classic example is SiH₄, which ignites spontaneously in air whereas GeH₄ is stable. Tin hydrides (SnH₄), called stannanes, are less stable and decompose slowly in air to tin and hydrogen gas. PbH₄ has not been isolated or prepared so far, and only hydrides containing organic substituents have been observed.

3. Halides

Group 14 elements generally form MX₄-type compounds, but germanium, tin and lead also form compounds of the type MX₂. Tetrachlorides of silicon and germanium are important precursors and are used in the syn-thesis of ultrapure silicon and germanium. These find applications in the electronic industry as materials for semiconductors. Silicon tetrachloride instantly hydrolyses when it comes into contact with water.