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Chemical Bonding and Geometry

There is a sharp distinction between ionic and covalent bonds when the geometric arrangements of atoms in compounds are considered. In essence, ionic bonding is nondirectional, whereas covalent bonding is directional. That is, in ionic compounds there is no intrinsically preferred direction in which a neighbour should lie for the strength of bonding to be maximized. In contrast, in a covalently bonded compound, the atoms adopt specific locations relative to one another, as in the tetrahedral arrangement of hydrogen atoms around the central carbon atom in methane, CH4, or the angular arrangement of atoms in H2O.

The lack of directionality of ionic bonds stems from the isotropy (spherical symmetry) of the electrostatic forces between ions. As has already been pointed out, the result of this isotropy is that ions stack together in the locations necessary to achieve the lowest energy and in this way give rise to the common packing patterns characteristic of many ionic solids. When deviations from stacking schemes are observed that seem to indicate that the ions are being held in certain orientations relative to their neighbours, it is a sign that covalent bonding is beginning to influence the structure of the solidand that the bonding is not purely ionic. This is the case, for example, in the compound nickel arsenide (NiAs), which has a structure that suggests that a degree of covalent bonding is present (Figure 6). It is fully apparent in the structure of diamond (Figure 7), in which each carbon atom is in a tetrahedral position relative to its neighbour and in which the bonding is essentially purely covalent.

nickel arsenideFigure 6: The crystal structure of nickel arsenide. This type of structure departs strongly from that expected for ionic bonding and shows the importance of covalence. There is also some direct nickel-nickel bonding that tends to draw the nickel atoms together.Encyclopædia Britannica, Inc.diamondFigure 7: The crystal structure of diamond. Each carbon atom is bonded covalently to four neighbours arranged tetrahedrally around the central atom. The structure is highly rigid.Encyclopædia Britannica, Inc.

The rationalization of the structures adopted by purely ionic solids is essentially a straightforward exercise in the analysis of electrostatic interactions between ions. The problem of the structures of covalent compounds, both individual molecules, such as methane, and covalently bonded solids, such as diamond, is much more subtle, for it involves delving into the characteristics of the electron arrangements in individual atoms. Thus, if the formation of a covalent bond is regarded as corresponding to the accumulation of electrons in a particular region of an atom, then, to form a second bond, electrons can be accumulated into only certain parts of the atom relative to that first region of enhancedelectron density. As a result, the bonds will lie in a geometric array that is characteristic of the atom. The remainder of this section focuses on this problem, but a detailed quantum mechanical analysis is required for a full understanding of the matter.