Types of Bonds Covalent, Ionic, Polar, Metallic

2 Types of Bonds Covalent, Ionic, Polar, Metallic 

The traditional classification of chemical bonds into ionic, covalent, donor-acceptor, metallic and van der Waals corresponds to extreme types, but a real bond is always a combination of some, or even all of these types (Fig. 2.1). Purely covalent bonding can be found only in elemental substances or in homonuclear bonds in symmetric molecules, which comprise a tiny fraction of the substances known. Purely ionic bonds do not exist at all (although alkali metal halides come close) because some degree of covalence is always present. Nevertheless, to understand real chemical bonds it is necessary to begin with the ideal types. In this Section we will consider mainly the experimental characteristics of different chemical bonds and only briefly the theoretical aspects of interatomic interactions. 

---

Types of Bonds Covalent, Ionic, Polar, Metallic Table 2.6 XRD effective atomic charges in binary compounds... 

From the scientific viewpoint, most materials have bonding that cannot be considered to be purely covalent or ionic (or metallic). Rather, these types of bonding are seen as ideals or prototypes, which provide useful categories for classifying many bonds to a first approximation . In most materials, the bonding may be more precisely described as intermediate, with varying degrees of covalent and ionic (and metallic) character. The notion of bond polarity indicates that the covalent-ionic dimension should be seen as a continuum, and not a dichotomy. 

If covalent, ionic and metallic bonds are explained in electrical terms, students are better prepared to accept that hydrogen bonds, van der Waals forces, solvent-solute interactions etc. are also types of chemical bonding. Where learners see covalent bonds as electron pairs attracted to two different positive cores, they have a good basis for subsequently learning about electronegativity and bond polarity. 

The types of interactions found in substances such as wax, polythene and sulfur, often called van der Waals forces, are not the only important types of bonding beyond metallic, ionic and covalent bonds. Solvent-solute interactions may be due to transient dipoles, but can also often involve permanent (rather than just transient) polarity on molecules. This is why water is a good solvent for ionic materials. 

So far, we have described chemical bonds as either ionic, when they occur between a metal and a nonmetal, or covalent, when they occur between nonmetals. In fact, ionic and covalent bonds are simply the extremes in a spectrum of bonding. Bonds that fall between these two extremes are polar, meaning that electrons are shared but are not shared equally. Such bonds are referred to as polar covalent bonds. The following shows a comparison of the different types of bonds, where M and X represent two different elements ... 

The forces involved in the interaction al a good release interface must be as weak as possible. They cannot be the strong primary bonds associated with ionic, covalent, and metallic bonding neither arc they the stronger of the electrostatic and polarization forces that contribute to secondary van der Waals interactions. Rather, they are the weakest of these types of forces, the so-called London or dispersion forces that arise from interactions of temporary dipoles caused by fluctuations in electron density. They are common to all matter. The surfaces that are solid at room temperature and have the lowest dispersion-force interactions are those comprised of aliphatic hydrocarbons and fluorocarbons. 

The difference in chemical behavior between metals and nonmetals is intuitively clear to any chemist. Theoretical chemistry describes this diversity in terms of different types of chemical bonds. They are portrayed in textbooks as being nonpolar covalent, polar covalent, ionic, dative, donor-acceptor, coordination, and so on. Chemists ascribe specific bonds to the above types without a clear explanation of the grounds... 

Evidently many crystals contain bonds of two or more quite distinct types. In molecular crystals consisting of non-polar molecules the bonds within the molecule may be essentially covalent (e.g. 85 or Sg) or of some intermediate ionic-covalent nature (e.g. Sip4), and those between the molecules are van der Waals bonds. In a crystal containing complex ions the bonds within the complex ion may approximate to covalent bonds while those between the complex ion and the cations (or anions) are essentially ionic in character, as in the case of NaNOg already quoted. In other crystals there are additional interactions between certain of the atoms which are not so obviously essential as in these cases to the cohesion of the crystal. An example is the metal-metal bonding in dioxides with the rutile structure, a structure which in many cases is stable in the absence of such bonding. 

Ionic bonds are described as well. The transition from covalent over polar covalent to ionic bonds is veiy fluent and depends on the difference in electronegativity between the atoms. The covalent bonds consist of sharing an electron pair and ionic bonds are electrostatic interactions between a cation and an anion. Solid ionic compounds are often arranged in lattice structures with many similarities with the lattice structures that we saw for the metallic compounds. The type of lattice structure for solid ionic compound depends on the ration between the radius of the cation and anion. 

LiB has been so far unknown. We think they probably have a polar-covalent type of the chemical bonds, which is similar to the ionic type. In the LiCl melt, which contains a certain amount of free delocalized electrons, the said borides may dissociate to lithium cations and boron anions having the hypothetical composition B . The latter may serve as the boron carriers from pure boron or the higher lithium borides to the metal particles. As a result, thermodynamically stronger borides of refractory metals are formed through diflhision. 

---