Dr. Anthony Haynes

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CHM1001.4: Hydrogen and the s- and p-block elements

Solid state structures of the p-block elements

Group 13

Boron Boron can adopt several solid state structures. The simplest contains linked icosahedral B12 units. A single B12 unit is shown opposite. The icosahedron has very high symmetry, with 5-fold, 3-fold and 2-fold rotation axes.  Can you spot them?
Aluminium is a metallic solid with a face-centred cubic (FCC) structure (also called cubic-close-packed, CCP). Note the relationship between this cubic structure and that of diamond shown below.

Group 14

Carbon Cn (diamond) The diamond structure is a face centred cubic lattice of carbon atoms with half of the tetrahedral holes also occupied by carbon atoms. This results in an immensely strong covalent network, in which each carbon is bound to four others, arranged at the corners of a tetrahedron  The same structure is taken up by silicon germanium and grey tin (α-Sn). Diamonds are formed naturally under extreme pressures deep within the earth, but synthetic diamonds can also be made.
Carbon Cn (graphite) Graphite has 2-dimensional layers of carbon atoms arranged hexagonally. Each C atom is trigonal and considered sp2 hybidised, with delocalised pi-bonding across each sheet facilitating electrical conductivity.
Carbon  C60 (buckminsterfullerene) The C60 structure can be derived from an icosahedron like B12 shown above. To generate the C60 structure, each of the 12 vertices of an icosahedron is truncated (sliced off) to leave a pentagonal face. An article reviewing 20 years since the discovery of C60 has appeared in the December 2005 issue of Chemistry World
Carbon  C70 (another fullerene)

Group 15

Phosphorus P4 (white phosphorus) This form of phosphorus contains tetrahedral P4 molecules.  The strained 60∘ bond angles lead to relatively high reactivity (e.g. with oxygen to give P4O6 and P4O10).
Phosphorus Pn (black phosphorus) The orthorhombic form of black phosphorus contains non-planar 6-membered rings of P atoms linked into layers. The model opposite shows part of one such layer, along with other atoms from adjacent layers. The rhombohedral polymorph of black phosphorus also contains 6 membered rings but with a different geometry (see N.C. Norman, p37).

Group 16

Sulfur (molecular); S8 The monoclinic and rhombic polymorphs of sulfur both contain S8 molecules as shown opposite. The two polymorphs differ in the way that the S8 molecules are packed together in the solid state crystal structure
Sulfur (helical, polymeric); Sn Also sometimes known as "plastic sulfur" this allotrope of sulfur consists of long chains of S atoms. At room temperature it slowly converts to rhombic sulfur with S8 molecules as shown above.

Group 17

Iodine I2  (solid) The solid state structure of iodine, is composed of a regular array of I2 molecules. However, it can also be related to the structures of elements from groups 13 (α-Ga) 14 (diamond) 15 (rhombohedral black P) and 16 (S or Se chains) by simply changing the spacing between the atoms (see N.C. Norman, p39-40)