P.A. Cox - Inorganic chemistry (793955), страница 23
Текст из файла (страница 23)
1. Normal boiling points of some molecular hydrides, with noble gas elements for comparison.In addition to forces of a strictly nonbonding nature, molecules may have chemical interactions that contribute to theapparent intermolecular forces.
Donor and acceptor centers on different parts of a molecule can lead to self-associationand polymerization, as discussed in Topic C8. Hydrogen bonding is one manifestation of this type of interaction (see96C10—MOLECULES IN CONDENSED PHASESTopic F2), which is especially important in polar hydrides of period 2 elements, NH3, H2O and HF. The extent towhich the boiling points of these compounds are out of line as a consequence can be seen in Fig. 1. Hydrogen bondingcan also have an important influence on the structure of liquids and solids: thus ice has structures in which each watermolecule is hydrogen bonded to four others.Molecular polarityThe polarity of a molecule is measured by its dipole moment µ: imagine charges +q and −q separated by a distance d,then, by definition, µ=qd.
A practical unit for µ at the molecular level is the Debye (D), equal to 3.336×10−30 C m.Polarity is a measure of charge separation in bonds and can often be related to the electro-negativity difference betweenthe atoms, as can be seen in the following series of µ/D values: HF (6.4); HCl (3.6); HBr (2.7); HI (1.4).
There are,however, reasons why a unique correlation of dipole moments with electronegativity differences is impossible, as follows.• Lone-pair electrons also have dipole moments. Sometimes these may reinforce the moment resulting fromelectronegativity difference (as in H2O and NH3, which both have very large net dipoles); in other cases the twocontributions may oppose each other, as happens in CO where the net dipole is very small.• In polyatomic molecules the dipoles associated with each bond add vectorially to give a resultant that depends on thebond angles.
In highly symmetrical molecules such as BF3, CF4, PF5 or SF6 the net dipole moment is zero eventhough the individual bonds may be strongly polar. As discussed in Topic C3, only certain point groups are compatiblewith an overall dipole moment.Larger dipoles lead to stronger intermolecular forces although other factors (e.g. hydrogen bonding) can also beinvolved. For molecular species such as LiF(g) the dipole moment is very nearly that predicted for an ionic Li+F− chargedistribution. Such ionic species do not, however, condense to form molecular solids or liquids, but rather ones withtypical ionic structures where each anion is surrounded by several cations and vice versa (see Topic D3).Application of an electric field to a substance causes a partial alignment of polar molecules; thus molecular dipolemoments contribute to the dielectric constant of a liquid or solid, one of the most important physical propertiesdetermining the behavior of a solvent (see Topic E1).
In nonpolar substances the dielectric constant arises from themolecular polarizability, and is generally much smaller than with polar molecules.Section D—Structure and bonding in solidsD1INTRODUCTION TO SOLIDSKey NotesCrystals and glassesLooking at unit cellsNonstoichiometryChemical classificationRelated topicsA crystalline solid is characterized by a unit cell containing anarrangement of atoms repeated indefinitely; noncrystalline or glassysolids do not have a unit cell. Short-range order resulting from thelocal bonding of atoms may, however, be similar in crystals and glasses.Different representations of unit cells are possible. It is important tounderstand how to use them to determine the stoichiometry of thecompound and the coordination of each atom.Some solids, especially natural minerals and transition metalcompounds, are nonstoichiometric with variable composition.The classification of solids into molecular, metallic, covalent(polymeric) and ionic types is useful provided it is recognized thatthere are no hard boundaries between them.Electronegativity and bondMethods of characterizationtype (B1)(B7)Crystals and glassesUnlike a molecule or complex ion, which is a finite (often small) assembly of atoms, a solid has no fixed size but can addatoms indefinitely.
In a sample of uniform composition the bonding arrangements of atoms are expected to be similarthroughout. For example, both crystalline and glassy forms of silica (SiO2) have structures with each Si surrounded byfour oxygen atoms, and each O by two Si. However in crystalline solids it is possible to identify a unit cellcontaining a group of atoms that is repeated indefinitely in precise geometric fashion.
In practice, all crystals containdefects where this regularity is broken sometimes, but nevertheless crystals are different from non-crystalline solidsor glasses, where there is no regular repetition. The crucial distinction is that of long-range order. Local chemicalbonding arrangements determine short-range order, which may be present even in a glass as in the case of SiO2. Thedifference arises from the way these bonds connect together to form an extended network. Glassy forms aremetastable, prevented by kinetic factors from achieving the most stable (crystalline) arrangement.
Very many, possiblyall solids can be made glassy if they are cooled rapidly enough from the gaseous or liquid state. Some substances formglasses especially easily, commonly ones in which atoms are covalently bonded to relatively few (three or four)neighbors.Macroscopically the distinction can be observed in the definite shapes of crystals, which reflect the regular atomicarrangements: compare, for example, the cubic faces of common salt (sodium chloride) crystals with the irregular andoften curved surfaces of fractured window glass. Microscopically the distinction can be made by X-ray diffraction,which depends on the fact that the regular atomic spacing in crystals is similar to the wavelength of X-rays (seeD1—INTRODUCTION TO SOLIDS99Topic B7).
This is the most powerful technique for determining the structures of crystalline substances but cannot beused in the same way for glasses.A ‘complete’ specification of the structure of a glass is impossible, but for a crystal it is only necessary to give thedetails of one unit cell. Substances are said to have the ‘same’ structure if the arrangement of atoms within a unit cell isessentially similar, although the interatomic distances and the dimensions of the cell are different.
Structure types arenamed after a particular example, frequently naturally occurring minerals: thus we talk of the rocksalt structure ofNaCl or the rutile structure of TiO2. Specifying a definite mineral rather than the compound formula is important, assome compounds show polymorphism and can adopt several different crystal forms. TiO2, for example, is knownalso as brookite and anatase, in which the arrangement of atoms is different from that in rutile.Looking at unit cellsTo specify the complete structure of a crystalline solid it is only necessary to show one unit cell, but interpreting thesepictures requires practice. Figure 1 shows some views of the cesium chloride structure (CsCl, depicted as MX).(a) Figure 1a is a perspective view (more correctly known as a clinographic projection), which is the mostcommon way of showing a unit cell.(b) Figure 1b shows a projection down one axis of the cell.
The position of an atom on the hidden axis isgiven by specifying a fractional coordinate (e.g. 0.5 for the central atom showing it is halfway up). Nocoordinate is given for atoms at the base of the cell.(c) Figure 1c shows the atoms shifted relative to the unit cell, and emphasizes the fact that what is importantabout a unit cell is its size and shape; its origin is arbitrary because of the way in which it is repeated to fillspace.(d) In Figure. 1d the drawing has been extended to show some repeated positions of the central atom.
Thishelps in seeing the coordination of the corner atom (see below).The most important aspect of any structure is its stoichiometry, the relative numbers of different types of atoms. Thestoichiometry of a unit cell can be determined by counting all the atoms depicted, and then taking account of those thatare shared with neighboring cells. Any atom at a corner of a unit cell is shared between eight cells, any at an edgebetween four, and any on a face between two. Thus the composition MX in Fig. 1 is arrived at by counting the eightcorner M atoms, and then dividing by eight to account for sharing. With some experience, this procedure will seemunnecessary. If one simply imagines the unit cell with a shifted origin as in Fig.
1c then it is immediately clear that everycell contains one M and one X atom.Another feature characteristic of a structure is the coordination of each atom. There is usually no difficulty inseeing the coordination of an atom in the middle of a unit cell. (For example, X in Fig. 1a can easily be seen to have eightM neighbors forming the corners of cube. In the projection, Fig. 1b. one needs to remember that the M atoms at thebase of the cell are repeated at the top.) For atoms at corners or edges it is necessary to consider what happens inneighboring cells, and an extended drawing such as Fig.
1d may be helpful: this shows each M surrounded by eight Xneighbors in the same way as the coordination of X.NonstoichiometryWhereas a pure molecular substance has a definite stoichiometry, this is not always true for solids. Defects in crystalscan include vacancies (atoms missing from their expected sites) and interstitials (extra atoms in sites normally100SECTION D—STRUCTURE AND BONDING IN SOLIDSFig. 1. Alternative views of the CsCl structure (see text).vacant in the unit cell).
An imbalance of defects involving different elements can introduce nonstoichiometry. This iscommon in compounds of transition metals, where variable oxidation states are possible (see Topics D5 and H4). Forexample, the sodium tungsten bronzes are formulated as NaxWO3, where x can have any value in the range 0–0.9.Another form of nonstoichiometry arises from the partial replacement of one element by another in a crystal. It iscommon in natural minerals, such as the aluminosilicate feldspars (Na,Ca)(Al,Si)4O8. The notation (Na,Ca) means thatNa and Ca can be present in the same crystal sites in varying proportions.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
