Elemental substances structures

The most familiar metals are elemental substances such as iron, tin, aluminium etc. However, many compounds are metallic. As well as intermetallic compounds such as AgCd and NaTl, and a huge number of non-stoichiometric alloys, many oxides, sulphides, halides etc. have metallic properties. For details of structure and bonding in metallic substances, see Section 7.5. 

Typical metallic substances have structures which cannot readily be described either in terms of directional covalent bonds or as arrays of cations and anions. Metallic elemental substances exhibit high coordination numbers, and can - to a first approximation - be viewed as arrays of cations embedded in a sea or glue of electrons completely delocalised over the crystal. This model helps to explain the characteristic mechanical, thermal and electrical properties of metals. It is also consistent with... 

The atomic radius of the atom X is defined as half the length of an X-X single bond. This can be obtained experimentally from the structures of elemental substances containing molecules X where the X-X bond order is believed to be unity, e.g. Cl2, P4, S8. It may also be obtained from the X-X distances found in molecules such as HO—OH, H2N—NH2 etc. for atoms which form multiple bonds in the elemental substance. Such atomic radii may be termed covalent radii. For atoms which form metallic elemental substances, metallic radii are obtained. These are usually standardised for 12-coordination of each atom, which is the most common situation in metals. Corrections can be made in the cases of metals which adopt other structures. 

The majority of elemental substances, and a large number of compounds, have metallic properties (see Section 3.3). Metallic elemental substances are characterised by three-dimensional structures with high coordination numbers. For example, Na(s) has a body-centred cubic (bcc) structure in which each atom is surrounded by eight others at the corners of a cube, each at a distance of 371.6pm from the atom at the centre. The Na atom also has six next-nearest neighbours in the form of an octahedron, with Na-Na distances of 429.1pm. A fragment of this lattice is shown in Fig. 7.14. These distances may be compared with the Na-Na bond length of 307.6 pm in the Na2 molecule, which can be studied in the gas phase by vaporisation of sodium metal. 

The atomisation enthalpies of the lanthanides as metallic elemental substances exhibit very different trends. From La to Eu, we see a steady decrease, followed by an abrupt increase at Gd. The atomisation enthalpies then decrease (not quite monotonically) to Yb, then increase at Lu. These trends may be rationalised as follows. According to magnetic studies, the lanthanide atoms in the elemental substances have the electronic configurations 6s25d14f" Eu and Yb are exceptions, discussed further below. The band structure is evidently complex and will not be described in detail. The atomisation enthalpy can be broken down for thermochemical purposes into two steps ... 

If the bands overlap, we have a metallic conductor because there is now in effect one partly-filled band. Compounds like CuZn represent this extreme case. Evidently there is a gradation among three-dimensional binary compounds between the extremes of ionic and metallic solids. As we shall see in Section 7.6, there is a similar gradation among elemental substances between extended covalent structures and typical metals. 

Apart from the atomic noble gases, elemental substances may be classified as metallic or covalent, according to their structures and properties at room temperature and ambient pressure. Covalent elemental substances may be subdivided as molecular or non-molecular, the latter category including one-, two- or three-dimensional structures. There is a grey area between the extremes of the three-dimensional covalent structure and the typical metal semi-metallic or metalloid behaviour is found in a number of cases. Even iodine, prima facie a molecular solid, has incipient metallic properties. In this section, we explore this grey area and consider the factors that determine which type... 

In order to illustrate the gradation from covalent to metallic behaviour we look at the structures and properties of the Group 14 elemental substances. 

Silicon and germanium as elemental substances are found only in the diamond-type form. The reluctance of Si and Ge to enter into pre-p bonding prohibits a graphite-type structure as a plausible allotrope. These are rather more reactive than diamond the weaker Si-Si and Ge-Ge bonds make disruption of the lattice kinetically easier. Tin occurs in both a metallic form (white tin) and a covalent (diamond-type) form the latter is slightly more stable at low temperatures. Lead forms only a metallic elemental substance. 

The metallic lustre of the elemental substances formed by the heavier Group 14 elements in the diamond structure can be interpreted in terms of the valence band/conduction band picture. The spectrum of excited states which can arise from promotion of an electron from the valence band to the conduction band covers the whole of the visible region, leading to opaqueness and specular reflectance. In the case of diamond itself, the lowest electronic excited state lies well into the ultraviolet. 

The reader is invited to consider other possible allotropic modifications of the Group 14 elemental substances, and to become satisfied that none is likely to be more stable than the structures observed. 

What factors determine whether an elemental substance adopts a metallic or a covalent structure From the simple model for metallic bonding, which views a metal as a lattice of cations embedded in a sea of delocalised electrons, it may be supposed that atoms having low ionisation potentials are most likely to become assembled as metallic substances. This correlation is far from perfect, however. Thus the first and second ionisation energies of mercury are comparable with those of sulphur, but the alchemists viewed elemental mercury and sulphur as the quintessential metal and nonmetal respectively. A closely-related correlation can be found with electronegativity. 

For any elemental substance, we can envisage both a metallic form and a covalent form perhaps several covalent forms. The most stable structure under ordinary conditions will be the winner in a thermodynamic... 

We may now summarise the features of an atom E which will favour the formation of a metallic lattice for the elemental substance in preference to a molecular structure. A metallic structure is favoured if ... 

The bonding can be quite simply described in terms of two-centre, electron-pair bonds. Re(III) has the d4 configuration of its nine valence orbitals - five 5d, one 6s and three 6p - five will be needed for the five Re-C1 bonds formed by each Re atom. Thus four electrons and four orbitals are available on each Re atom for M-M bonding, so that three Re=Re double bonds can be formed. The short Re-Re bond (248 pm, compared with 276 pm in the elemental substance) is consistent with a bond order of 2. The structure of Re3Cl9 is similar, with chlorine bridges joining the Re3 clusters. 

TAS° at 298 K is +3 kJ mol-1. In the case of LiH, an ionic solid having the NaCl structure, a thermochemical analysis (see Chapter 5) can rationalise its thermodynamic stability relative to the elemental substances. However, AG for LiH is much less negative than for LiF or LiCl (—616... 

Humic substances are ubiquitous in the environment, occurring in all soils, waters, and sediments of the ecosphere. Humic substances arise from the decomposition of plant and animal tissues yet are more stable than their precursors. Their size, molecular weight, elemental composition, structure, and the number and position of functional groups vary, depending on the origin and age of the material. Humic and fulvic substances have been studied extensively for more than 200 years however, much remains unknown regarding their structure and properties. 

Thus the salt water can be converted into four simple substances hydrogen, oxygen, sodium, and chlorine (Figure 2.6 on the next page). Chemists are unable to convert these four substances into simpler ones. They are four of the building blocks of matter that we call elements, substances that cannot be chemically converted into simpler ones. (We will get a more precise definition of elements after we have explored their structure in more detail.)... 

This structure does not account for the nitrogen and sulfur content of humic substances. It has been suggested that these elements are derived from parts of other types of molecule, for example, proteins, which are associated with the humic substances. Indeed it has been proposed that humic substances consist of an aromatic core to which peptides, carbohydrates, metals, and phenolic acids are chemically or physically attached. It can be seen that the structure in Fig. 5-9 is an "open" network. In fact, there have been suggestions that organic and inorganic materials associated with humic substances are trapped inside these "holes" in the humic substance structure. " ... 

Comparison of the decomposition products with the crystal structures of the corresponding substances shows that the equilibrium composition of products is observed only for white phosphorus, having the cubic structure. For elements with structures other than cubic, evaporation occurs with the formation (partial or complete) of binary molecules. Thus, the distinctive feature of crystals with cubic structure to sublime with the formation of equilibrium primary products manifests itself for all the substances considered above oxides, nitrides and white phosphorus. 

Detectors which are sensitive to specific elements or structures, such as electron capture detectors (ECD) or phosphorus- and nitrogen-selective detectors (TSD) respond even to quantities of substances in the picogram (10"12 g) Qj. femtogram (10 g) ranges. 

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