The Group 14 elements
Group 14 periodicity
This article addresses the periodicity displayed by the Group 14 elements but excluding, largely, ununquadium (element 114) about which virtually nothing is known. One could predict the properties of ununquadium based upno those of the higher elements and this is left as an exercise for the reader.
Nature of the elements
The elements become increasingly metallic down the group. Carbon, at the top, is a typical non-metal while silicon is a semiconductor profoundly important to the electronics industries. Tin and lead are very metallic although one modification of tin known as grey tin has the same diamond structure as does germanium and silicon. The elements lower down the group form complexes while carbon does not. The melting points of the elements decrease down the group as the elements become increasingly metallic.
Carbon often forms multiple bonds, both with itself (as in ethene and ethyne) and with other elements such as oxygen (as in carbon dioxide and ketones). In contrast, silicon, germanium, and tin only form analogues of ethene (albeit non-planar) when the elements possess bulky substituents. While the C=C π-bond formed through the overlap of C 2p-orbitals is strong, those lower down the group are much less strong. This also explains why graphite is stable while there are no analogues of graphite lower down the group. Carbon dioxide, CO2, possesses two carbon-oxygen double bonds (O=C=O) while the corresponding silicon dioxide, SiO2, possesses an extended lattice structure. This is because the π-bond formed through the overlap of p-orbitals on carbon and oxygen is strong as the overlap is favourable, while lower down the group the π-overlap is less efficient.
The hydrides MX4 are known for all the elements except ununquadium although the lead compound (plumbane, PbH4) is poorly characterized. Each is a covalent molecule. The parent hydride for carbon is methane, CH4, and there is an extensive range of compounds called alkanes of the type CnH2n+2 (methane, ethane, propane, butane....). There are relatively few of the corresponding silicon hydrides (silanes) and they are spontaneously flammable. The germane GeH4 is known while the stannane SnH4, a colourless gas, decomposes to tin at about 0°C.
Two types of halide for this group are known: MX2 and MX4. The M(IV) halides dominate the top of the group while the M(II) halides dominate at the bottom. All the M(IV) halides MX4 (M = C, Si, Ge; X = F, Cl, Br, I) are all known for the three elements carbon, silicon, and germanium at the top of the group. However, as the group is descended, the stability of the M(II) state increases relative to the M(IV) state. None of the dihalides MX2 exist independently for carbon or silicon while most of the divalent halides MX2 are known for germanium in addition to the germanium tetrahalides. At the bottom of the group the most stable lead halides are PbX2 and the only known tetrahalide seems to be PbCl4 (this decomposes exothermically to PbCl2 and chlorine gas).
- Carbon: 1086.5 kJ mol-1
- Silicon: 786.5 kJ mol-1
- Germanium: 762 kJ mol-1
- Tin: 708.6 kJ mol-1
- Lead: 715.6 kJ mol-1
Carbon has the highest ionization energy in the group. The ionization energy for silicon is lower because the outermost electrons for silicon (3p) are further away from the nucleus than those of the 2p level for carbon. The 3p electrons are further away at silicon because of the higher principal quantum number and because of screening by the n=1 and n=2 level electrons.
The downwards trend continues for germanium, but less so. This is a consequence of the d-block elements within the periodic table. There are ten extra charge units on the nucleus because of these 3d elements and because the ten 3d electrons do not screen the nucleus that efficiently, the effective nuclear charge is higher than it would have been without the 3d-elements. Consequently, electrons from the 4p level are more difficult to remove and the ionization energy of germanium is therefore little different than that of silicon. The ionization energy of tin is a little higher than it might have been because of the inclusion of the 4d elements. The first ionization energy of lead is actually higher than that of tin rather than lower. This is because of the inclusion not only of the 5d elements but also the 4f elements (the lanthanoids). The 4f electrons screen the nucleus rather inefficiently from 6p electrons causing the effective nuclear charge to be quite high, to the extent that the ionization energy for lead is actually a little higher than that of tin.