Close packed arrangements

Figure 6.2 Basal plane of a-rhombohedral boron showing close-packed arrangement of B 2 icosahedra. The B-B distances within each icosahedron vary regularly between 173-179 pm. Dotted lines show the 3-centre bonds between the 6 equatorial boron atoms in each icosahedron to 6 other icosahedra in the same sheet at 202.5 pm. The sheet-s are slacked so that each icosahedron is bonded by six 2-centre B-B bonds at 171 pm (directed rhombohedral ly, 3 above and 3 below the icosahedron). B12 units in the layer above are centred over 1 and those in the layer below are centred under 2.

The metals are silvery in appearance (except for Eu and Yb which are pale yellow, see p. 112 and below) and are rather soft, but become harder across the series. Most of them exist in more than one crystallographic form, of which hep is the most common all are based on typically metallic close-packed arrangements, but their conductivities are appreciably lower than those of other close-packed metals. 

Figure 11.2 Surface activity on the waterside of a condensate line, showing filmwise condensation of water on an unprotected area (a) and dropwise condensation on an area protected by a closely packed arrangement of filming amine molecules (b).

Brookite is also based upon a double hexagonal close-packed arrangement of the oxygen ions. The dimensions of the unit of structure differ so much on account of distortion from those for the ideal arrangement, however, that the existence of dose-packing was recognized only after the structure had been determined. 

F1G. 1. Two-dimensional representation of a nearly spherical close-packed arrangement of spherons, with one in the inner core (left), and of an arrangement with prolate deformation, consequent to having two spherons in the inner core (right). 

The low solubility of AuCN(s) is thought to be a consequence of a polymeric structure based on linear chains, —Au-CN-Au-CN—, which lie parallel to one another with a close-packed arrangement of gold atoms in which each is in contact with six nearest neighbors.36 The addition of cyanide and dissolution of [Au(CN)2] is believed to take place at the chain ends. This process is enhanced by the presence of Ag2+ or other ions such as Pb2+ (see below) normally present in the gold-bearing ores. 

The nickel crystal has a cubic close packing arrangement with an edge length of 352.4 nm. Using this information, calculate the density of nickel. 

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