Bond Type and Atomic Arrangement

X one of the states is the more stable, and represents the normal complex, and for others the other state is the stable one. At the discontinuity in the nature of the normal state of tne complex the energy curves of the two states cross. An actual system would contain complexes in both states, with concentrations determined by the energy difference of the two an appreciable number of complexes in the less stable state would be present, however, only for the region near the intersection of the two curves. 

These complexes and others of similar character are discussed further in Chapter 5, in which a magnetic criterion for bond type applicable to complexes of the transition elements is described. 

The properties of a substance depend in part upon the type of bonds, between the atoms of the substance and in part upon the atomic arrangement and the distribution of the bonds. The atomic arrangement is itself determined to a great extent by the nature of the bonds the directed character of covalent bonds (as in the tetrahedral carbon atom) plays an especially important part in determining the configura- 

Since 1913 a great amount of information about the atomic arrangement in molecules and crystals has been collected.1 This information can often be interpreted in terms of the nature and distribution df bonds a detailed discussion of the dependence of interatomic distances and bond angles on bond type will be given in later chapters. 

7 A great amount of information about the structure of crystals has been obtained by use of the x-ray diffraction method. The diffraction of x-rays by crystals was discovered by Max von Laue in 1912. Shortly thereafter W. L. Bragg discovered the Bragg equation, and in 1913 he and his father, W. H. Bragg, published the first structure determinations of crystals. 

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In the case of PDB ligands, only part of the graph information is defined in the PDB file format sepcification bond types and atom hybridization states are missing and will later be derived from the three-dimensional arrangement of connected atoms. Connectivity information, however, is already present and can be used to seperate cyclic from non-cyclic molecule parts. Especially for planar rings, this separation is a prerequisite for geometry interpretation, because of... 

NMR spectra are basically characterized by the chemical shift and coupling constants of signals. The chemical shift for a particular atom is influenced by the 3D arrangement and bond types of the chemical environment of the atom and by its hybridization. The multiplicity of a signal depends on the coupling partners and on the bond types between atom and couphng partner. 

There is, to be sure, some correlation between bond type and type of atomic arrangement. Ionic crystals often possess a coordinated structure such that ionic bonds extend throughout the crystal, leading to low volatility. Another structural feature that leads to high melting points and striking hardness of crystals is the hydrogen bond between molecules (Chap. 12). 

Graphite is planar, with the carbon atoms arranged in a hexagonal pattern. Each carbon atom is bonded to three others, two by single bonds, one by a double bond. The hybridization is sp2. The forces between adjacent layers in graphite are of the dispersion type and are quite weak. A lead pencil really contains a graphite rod, thin layers of which rub off onto the paper as you write (Figure 9.13, p. 242). 

We have already learned that metals may be deformed easily and we have explained this in terms of the absence of directional character in metallic bonding. In view of this principle, it is not surprising that two-element or three-element metallic crystals exist. In some of these, regular arrangements of two or more types of atoms are found. The composition then is expressed in simple integer ratios, so these are called metallic compounds. In other cases, a fraction of the atoms of the major constituent have been replaced by atoms of one or more other elements. Such a substance is called a solid solution. These metals containing two or more types of atoms are called alloys. 

Sometimes the atomic arrangement of a crystal is such as not to permit the formulation of a covalent structure. This is the case for the sodium chloride arrangement, as the alkali halides do not contain enough electrons to form bonds between each atom and its six equivalent nearest neighbors. This criterion must be applied with caution, however, for in some cases electron pairs may jump around in the crystal, giving more bonds than there are electron pairs, each bond being of an intermediate type. It must also be mentioned that determinations of the atomic arrangement are sometimes not sufficiently accurate to provide evidence on this point an atom reported equidistant from six others may be somewhat closer to three, say, than to the other three. 

Ionic bonds may be fully as strong as covalent bonds, so that properties such as hardness, solubility, melting point, ionization in solution, and chemical character are not especially valuable criteria as a rule. Sometimes comparison of properties with those of compounds of known bond type permits reasonably certain conclusions to be drawn. Thus the similarity in physical properties as well as in atomic arrangement of SiC, AIN, and diamond suggests that all three substances contain covalent bonds. PbS is like FeS2, MoS2, etc. in properties rather than like CaS, so that it is improbable that PbS is an ionic substance. 

The properties of a compound depend on two main factors, the nature of the bonds between the atoms, and the nature of the atomic arrangement. It is convenient to consider that actual bonds approach more or less closely one or another of certain postulated extreme bond types (ionic, electron-pair, ion-dipole, one-electron, three-electron, metallic, etc.), or... 

There is, of course, a close relation between atomic arrangement and bond type. Thus the four single bonds of a carbon atom are directed toward the comers of a tetrahedron But tetrahedral and octahedral configurations are also assumed in ionic compounds, so that it is by no means always possible to deduce the bond type from a knowledge of the atomic arrangement. 

Fig. 12.4 (a) Off-[001] perspective view of the a-type orthorhombic structure of R5Tt4. Small and large spheres represent R and Tt, respectively. The p- and y-type structures also have similar atomic arrangements but with different Tt-Tt bonding behavior. 

While Lavoisier had established a rational system for naming elements and compounds, Frankland developed the system that we use today for writing chemical formulas and for depicting the bonds between the atoms in molecules. As Frankland synthesized more and more isomers, compounds with the same formulas but different molecular structures, he found traditional formulas confusing they showed the types and numbers of elements but provided no clue as to how the atoms were arranged inside the molecule. To remedy the problem, Frankland depicted the atoms in functional groups and drew lines between them to indicate the bonds between the elements. 

Structural formulas also provide information on the way the atoms are arranged and bonded to one another within a molecule. The structural formula of substances not only specifies the type of atoms and how many atoms of each type there are in the molecule of a compound it also provides an outline of the structure of the molecule, pinpointing exactly where each atom is located. Each element in a structural formula is represented by its symbol, and the bonds between atoms are indicated by lines connecting the symbols (see Fig. 60). Thus, structural formulas not only provide information on the type and number of atoms in a molecule of a substance but also depict the internal structure of the molecule of the substance. 

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