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.

Multiple bonds

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.

Hydrides

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 C_nH_2n+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.

Halides

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).

Oxides

Ionization Energy

In the standard form of the periodic table the s-block, p-block, and d-block elements are organised into 18 vertical columns called groups. These are labelled from 1 to 18 under current IUPAC numenclature.

Earlier labelling schemes (Trivial Group names)

For historical reasons some Groups have special names. Terms such as the "alkali metals" are in very common use whereas the term "pnictogens" is very much less common. Some of these special names are listed in the Table.