Metal close packing

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. 

Bluish-white lustrous soft metal closed-packed hexagonal system density 8.69 g/cm3 Brinnel hardness 21 melts at 321.1°C vaporizes at 767°C vapor pressure 5 torr at 455°C electrical resistivity 6.8 microhm-cm at 0°C insoluble in water. 

Silvery-white metal close-packed cubic crystals lattice constant 3.8394A at 20°C density 22.42 g/cm (highest among metals) melts at 2410°C vaporizes at 4,130°C hardness 6-6.5 Mohs electrical resistivity 4.71 j,ohm-cm Young s modulus 3.75 x 10 tons/in magnetic susceptibility 0.133 x 10 cm3/g thermal neutron absorption cross section 440 barns. 

Silvery-white metal close-packed hexagonal structure density 1.74 g/cm at 20°C, 1.57 g/cm3 at 650 C (hquid melt) melts at 650 C vaporizes at 1,090°C vapor pressure 5 torr at 678 C and 20 torr at 763 C electrical resistivity 4.46 microhm-cm at 20 C, 28.0 microhm-cm at 650 C (hquid melt) surface tension 563 dynes/cm at 681 C modulus of elasticity 6.5x10 Ib/sq in Poisson s ratio 0.35 thermal neutron absorption cross section 0.059 bam soluble in dilute acids. 

In carbides, carbon is bound to elements with lower or similar EN-values. We distinguish three types of carbides. The salt-like carbides with elements from groups 1, 2 and 3 are decomposed by water A14C3 +12 H20 — 4 Al(OH)3 + 3 CH4. In addition, there are the covalent carbides like SiC and B4C and a intermediate group with most transition metals. In the intermediate group C atoms are located in the octahedral cavities of metal close packings. The melting points vary from 3000 to some extreme values of about 4800 °C and their hardness lies between 7 and 10 on the Mohs scale. Furthermore, the... 

Gray metal close-packed hexagonal structure anisotropic high permeability to X-rays, mp 1287°. bp 2500° (extrapolated). d 1.8477. Heat capacity at constant pressure (30°) 0.437 cal/g/ C Walker el al, J. Chem. Eng. Data 7, 595 (1962). Latent heat of fusion 3.5 kcal/mole. Brinell hardness 60-125, Chemical properties similar to aluminum ... 

PHYSICAL PROPERTIES gray metal, close-packed hexagonal structure hard-light metal odorless high permeability to x-rays hard and brittle soluble in acids (except nitric) and... 

Average oxidation number of the metal Close-packed structures Cluster strucures... 

The crystal structures of nonmolecular sohds can often be described in simple terms with reference to the close packing of spheres. In particular, matty metals adopt the stractures of simple close packing of identical spheres. In the case of metals, close-packed stractures enable close contact between metal atoms and allow for the conduction of the outer electrons, which gives rise to basic metallic properties such as electronic and thermal conduction. 

It may be recalled (Section 3.12) that for metals, close-packed planes of atoms stacked on one another generate both FCC and HCP crystal structures. Similarly, a number of ceramic crystal structures may be considered in terms of close-packed planes of ions, as well as unit cells. Ordinarily, the close-packed planes are composed of the large anions. As these planes are stacked atop each other, small interstitial sites are created between them in which the cations may reside. 

The wave function T i oo ( = 11 / = 0, w = 0) corresponds to a spherical electronic distribution around the nucleus and is an example of an s orbital. Solutions of other wave functions may be described in terms of p and d orbitals, atomic radii Half the closest distance of approach of atoms in the structure of the elements. This is easily defined for regular structures, e.g. close-packed metals, but is less easy to define in elements with irregular structures, e.g. As. The values may differ between allo-tropes (e.g. C-C 1 -54 A in diamond and 1 -42 A in planes of graphite). Atomic radii are very different from ionic and covalent radii. 

Figure Bl.21.1. Atomic hard-ball models of low-Miller-index bulk-temiinated surfaces of simple metals with face-centred close-packed (fee), hexagonal close-packed (licp) and body-centred cubic (bcc) lattices (a) fee (lll)-(l X 1) (b)fcc(lO -(l X l) (c)fcc(110)-(l X 1) (d)hcp(0001)-(l x 1) (e) hcp(l0-10)-(l X 1), usually written as hcp(l010)-(l x 1) (f) bcc(l 10)-(1 x ]) (g) bcc(100)-(l x 1) and (li) bcc(l 11)-(1 x 1). The atomic spheres are drawn with radii that are smaller than touching-sphere radii, in order to give better depth views. The arrows are unit cell vectors. These figures were produced by the software program BALSAC [35]-...

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

In a pure metal the atoms of the solid are arranged in closely packed layers. There is more than one way of achieving close packing but it... 

We have seen that in a metal the atoms are close-packed, i.e. each metal atom is surrounded by a large number of similar atoms (often 12, or 8). The heat required to break up 1 mole of a metal into its constituent atoms is the heat of atomisation or heat of sublimation. Values of this enthalpy vary between about 80 and 800 kJ. for metals in their standard states these values indicate that the bonds between metal atoms can vary from weak to very strong. There is a rough proportionality between the m.p. of a metal and its heat of atomisation. so that the m.p. gives an approximate measure of bond strength. 

The transition metal structures consist of close-packed (p. 26) arrays of relatively large atoms. Between these atoms, in the holes , small atoms, notably those of hydrogen, nitrogen and carbon, can be inserted, without very much distortion of the original metal structure. to give interstitial compounds (for example the hydrides, p. 113). 

As with other related rare-earth metals, gadolinium is silvery white, has a metallic luster, and is malleable and ductile. At room temperature, gadolinium crystallizes in the hexagonal, close-packed alpha form. Upon heating to 1235oG, alpha gadolinium transforms into the beta form, which has a body-centered cubic structure. 

Spinel Ferrites. In spinel ferrites having the composition where A and B are metals, cubic close-packed oxygen ions leave two kinds of... 

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