P.A. Cox - Inorganic chemistry (793955), страница 24
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Simultaneous (Si,Al) replacement ensures thatall elements remain in their normal oxidation states. Even this formulation is approximate, as several other elementsmay be present in smaller proportions.Chemical classificationSolids are often classified according to their chemical bonding, structures and properties (see Topic B1):Molecular solids contain discrete molecular units held by relatively weak intermolecular forces (seeTopic C10).Metallic solids have atoms with high coordination numbers, bound by delocalized electrons that give metallicconduction.Covalent or polymeric solids have atoms bound by directional covalent bonds, giving relatively lowcoordination numbers in a continuous one-, two- or three-dimensional network.D1—INTRODUCTION TO SOLIDS101Ionic solids are bound by electrostatic attraction between anions and cations, with structures where everyanion is surrounded by cations and vice versa.Although these broad distinctions are useful, many solids show a degree of intermediate character, or even several typesof bonding simultaneously.
Metallic and covalent interactions both arise from overlapping atomic orbitals (seeTopics C4–C7) and the distinction in physical properties arises from the energy distribution of electronic levels (seeTopic D7). The structures and electronic properties of elements show a gradation in character at the metal-nonmetalborderline (see Topics B2 and D2). A similar gradation is seen between ionic and covalent compounds as theelectronegativity difference between two elements changes (see Topics B1 and D4).
Furthermore, solids withpredominantly ionic bonding between some atoms can also have covalent bonds between others (see Topic D5).Section D—Structure and bonding in solidsD2ELEMENT STRUCTURESKey NotesSphere packingMetallic elementsNonmetallic elementsRelated topicsSpheres of equal size may be packed in three dimensions to givehexagonal close-packed (hcp) and cubic close-packed (ccp, also knownas face-centered cubic, fcc) structures. The body-centered cubic (bcc)structure is slightly less efficiently close packed.Many metallic elements have hcp, fcc or bcc structures. There are someclear group trends in structure, although there are exceptions to theseand some metals have less regular structures, especially in the p block.Most nonmetallic elements have structures that can be understood usingsimple electron-pair bonding models. C, N and O can form multiplebonds and are exceptional in their groups.Chemical periodicity (B2)Introduction to nonmetals(F1)Sphere packingElement structures where chemical bonding is nondirectional are best introduced by considering the packing of equalspheres.
Close-packed structures are ones that fill space most efficiently. In two dimensions this is achieved in alayer with each sphere surrounded hexagonally by six others. Three-dimensional structures are developed by stackingthese layers so that the spheres in one layer fall over the hollows in the one below, as shown in Fig. 1a. Having placedtwo layers, labeled A and B, there are alternative positions for the spheres in the third layer.
They could be placeddirectly over spheres in the first layer A to give a sequence denoted ABA. Alternatively, the spheres in the third layercan be placed in positions where there are gaps in layer A; two such spheres labeled C are shown in Fig. 1a. A regularpacking based on this latter arrangement would then place the fourth layer directly over layer A, giving a sequence denotedABCA. The simplest three-dimensional close-packed structures are based on these two regular sequences of layerpositions:ABABABAB…gives hexagonal close packing (hcp);ABCABCABC…gives cubic close packing (ccp).These structures are illustrated in Fig.
1b and c. respectively. In the ccp arrangement, successive close-packed layers areplaced along the body diagonal of a cube. The unit cell shown is based on a cube with atoms in the face positions, andthe structure is also known commonly as face-centered cubic (fcc).D2—ELEMENT STRUCTURES103Fig. 1. Close-packed structures, (a) Stacking of layers showing the sequence ABC (see text); (b) the hcp structure; (c) one unit cell of the fcc structure.In both fcc and hcp structures each sphere is surrounded by 12 others at the same near-neighbor distance. (There are sixin the same close-packed layer, and three each in the layers above and below.) If the spheres are in contact bothstructures give 74% filling of space by the spheres, with the remaining 26% outside them.
This is the optimum spacefilling possible with equal spheres. Similarly close-packed structures can be constructed from more complicatedsequences of layers such as ABABCABABC…, or even with random sequences. Although these are sometimes found, mostclose-packed structures are of the simple fcc or hcp types.Another structure that gives fairly efficient space filling (68% compared with 74% above) is the body-centeredcubic (bcc) one illustrated in Fig.
2. Each atom has eight near-neighbors, but there are six others (also shown in thefigure) slightly further away.Fig. 2. Bcc structure.104SECTION D—STRUCTURE AND BONDING IN SOLIDSMetallic elementsA high proportion of metallic elements have one of the three structures ccp, hcp or bcc just described. The factors thatdetermine the structure are subtle. In some cases the thermodynamically stable structure depends on temperature and/or pressure, showing that the energy differences between them are small.
Nevertheless, some regularities are observedin the periodic table, which suggest that stability depends in a systematic way on the number of valence electrons. Thecommonest stable structures according to group number are1: bcc5, 6: bcc2: varied7, 8: hcp3, 4: hcp9–11: fccThere are irregularities, however. In the transition metal groups 7, 8 and 9 the 3d series elements Mn, Fe and Co areexceptions.
Some elements also have more complex structures, especially in the p block. An understanding of thefactors controlling metallic structures requires the band theory of delocalized electrons, not discussed in this book.Nonmetallic elementsAs might be expected from other aspects of its chemistry, boron is exceptional and has elemental structures that cannotbe understood in simple bonding terms (see Topic F3). For the remaining nonmetals, the simple concepts of electronpair bonding and stereochemistry described in Topics C1 and C2 allow the structures to be rationalized although notalways predicted.
Single-bonded structures where each element achieves an octet lead to the following predictions.Group 14:Group 15:Group 16:Group 17:Group 18:four tetrahedral bonds as shown in the diamond structure of C, Si, Ge and Sn, andillustrated in Fig. 3a.three bonds in a pyramidal (nonplanar) geometry, which can give rise to P4 molecules(white phosphorus) or a variety of polymeric structures shown by P and As (seeTopic F6).
Phosphorus has several allotropes, some with apparently complexstructures, but all are based on the same local bonding.two bonds, noncolinear, as found in S8 rings and in spiral chains with Se and Te (seeTopic F8). The different allotropes of sulfur all have this bonding.one bond, giving diatomic molecular structures shown by all the halogens (seeTopic F9).no bonds, leading to monatomic structures with atoms held only by van der Waals’forces (see Topics C10 and F10). The normal solid structure of the noble gas elementsis fcc.The structural chemistry of the period 2 elements C, N and O shows a greater tendency to multiple bonding than inlower periods (see Topics C8 and F1). Molecular N2 (triple bonded) and O2 (double bonded) are the normal forms ofthese elements.
With carbon, other allotropes in addition to diamond are possible. The thermodynamically stable format normal pressures is graphite (see Fig. 3b), where some delocalized π bonding is present along with the three a bondsformed by each atom. Fullerenes such as C60 have similar bonding arrangements (see Topic F4).D2—ELEMENT STRUCTURES105Fig. 3. Structures of (a) diamond and (b) graphite.Another group trend with p-block elements is the increasing tendency towards metallic character in lower periods. Aswith the chemical trends, the change in structures and properties of the elements appears more of a continuoustransition than a sharp borderline (see Topics B2 and D7).
The structural distinction between near-neighbor (bonded)atoms and next-near-neighbor (nonbonded) ones becomes less marked down each group. Table 1 lists the ratio of thesedistances for some nonmetallic elements of periods 3–5, and shows how the two distances become more nearly equal forheavier elements, especially with Sb and Te, which are close to the metallic borderline. The peculiar structures shownby some p-block metals suggests that some influence of directional bonding persists in the metallic state.Table 1. The ratio of next-near-neighbor to near-neighbor distances in some solid p-block elementsSection D—Structure and bonding in solidsD3BINARY COMPOUNDS: SIMPLE STRUCTURESKey NotesCoordination numberand geometryThe coordination number (CN) and geometry of atoms (or ions) arethe most important characteristics of a structure.
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