Distinction Between Ionic and Covalent Bonding

The word bonding applies to any situation in which two or more atoms are held together in such close proximity that they form a characteristic species which has distinct properties and which can be represented by a chemical formula. In compounds consisting of ions, bonding results from the attractions between the oppositely charged ions. In such compounds in the solid state, each ion is surrounded on all sides by ions of the opposite 

In contrast, covalent bonding involves the sharing of electron pairs between two specific atoms, and it is possible to speak of a definite bond. For example, in molecules of HCl and CH4 there are one and four covalent bonds per molecule, respectively. 

Polyatomic ions such as OH, CIOs , and NH4+ possess covalent bonds as well as an overall charge. 

The charges on polyatomic ions cause ionic bonding between these groups of atoms and oppositely charged ions. In writing electron dot structures, the distinction between ionic and covalent bonds must be clearly indicated. For example, the electron dot diagram for the compound NH4CIO3 is 

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Polar covalent bonds can be regarded as having some degree of ionic character, and the distinction between ionic and covalent bond types is sometimes hard to make. Some compounds have clear examples of both types of bonding simultaneously. Thus CaC03 has well-defined carbonate ions... 

Before discussing each type, we should recognize that the distinction between ionic and covalent bonding is not always clear-cut. Some compounds are clearly ionic, and some are clearly covalent, but many others possess both ionic and covalent characteristics. 

Two types of bonds, namely, ionic and nonionic (covalent) bonds, were recognized early on in the formulation of the electronic theory of valency. Lewis made clear distinctions between ionic and covalent bonds. The first are formed by transfer of electrons and production of separate charged ions (as explained by Kossel, 1916), the second, according to Lewis, by sharing of electrons in pairs, a single bond consisting of one shared pair, a double bond of two, and a triple bond of three. 

The short primary bonds that are responsible for the stereoactivity of the lone pairs on anions are normally described as covalent, but bmid valence theory makes no distinction between ionic and covalent bonds, since the bond valence is a variable that runs continuously across the whole spectrum of bond types. The ionic-covalent distinction does, however, reflect a marked difference between the structural chemistry of those compounds that obey the valence matching rule (9) in which the bonding is generally weak (less than about 0.8-1.0 vu), and those where the presence of anion lone pairs permits the formation of much stronger bonds by making the lone pairs stereoactive as described in Sect. 7.1. The bonds that obey the valence matching rule are those traditionally described as ionic, while bonds formed by atoms with stereoactive lone pairs are those traditionally described as covalent. Even though the bonds form a continuous series in which such a distinction is not necessary, the term covalent can be usefully applied to the primary (short) bonds formed by anions with stereoactive lone pairs. 

One of the many attractive features of bond valence theory is that it provides a single description of a bond that spans the whole range of bond types with valences ranging upwards from zero. Because bonds form a continuum, the theory makes no distinction between ionic and covalent bonds aU bonds obey the same set of rules. Yet the use of the terms ionic and covalent is so widespread that their usefulness cannot be questioned. Some might define covalent bonds as those that share bonding electrons and ionic bonds as those formed by the electrostatic... 

In the hydrated salts and in aqueous solution the nitrate ion is a distinct entity. This is also true for the anhydrous nitrates of the electropositive metals, but in many other salts the nitrate ion is bonded covalently to the metal. The distinction between ionic and covalent nitrates can most easily be made on the basis of their infrared spectra. Chasan and Norwitz9 prepared a listing of ionic and covalent salts based on an examination of the literature, and covalent nitrate complexes were reviewed by Addison and Sutton.5 Spectra of liquid nitrates were discussed by Wait and Janz.10 Figure 5.2 represents the author s evaluation of the literature with respect to the existence and bonding of anhydrous nitrates, but no attempt was made to classify the covalent nitrates with respect to the number of oxygens bonded to the metal atom. This topic, which is of great structural interest,... 

A full description is beyond the scope of this review, but it is noted that the topological method identifies other chemical features in the electron density. The union of all bond paths gives a bond path network that is normally in a 1 1 correspondence with the chemical bond network drawn by chemists. The bond paths for bonds in strained rings are curved, reflecting their bent nature. In Figure 6, we show the gradient paths in the molecular plane of cyclopropane. The C—C bond paths are distinctly bent outward. The value of the Laplacian at the bond critical point discriminates between ionic and covalent bonding." Maps of the Laplacian field reveal atomic shell structure, lone pairs, and sites of electrophilic and nucleophilic attack. The ellipticity of a bond measures the buildup of density in one direction perpendicular to the... 

The Bond Valence Model has its roots in the ionic models of Pauling, but it can be equally well derived from the covalent models of Lewis. It thus spans the full range between ionic and covalent bonds without making any distinction between them. In the formal development of the model, a chemical structure is treated as a network of bonds in which each bond is associated with a valence that expresses its strength. The two network equations, viz the Valence Sum Rule (Equation 10.5) and the Equal Valence Rule (Equation 10.6), can be used to predict bond valences, and hence bond lengths, when the bond network is known. The influence of one part of the structure on another is transmitted through the network by application of the network equations. 

The adsorbed species 0 is not believed to exceed --10% on the basis of the available capacity data. While these workers propose reaction (XX), they also point out that on the basis of the pH dependence alone it is not possible to distinguish between 0 and OH" adsorption. Vetter and Schultze indicate that the species is ionic rather than a covalently bonded species. A clear distinction between ionic vs. covalent bonding is difficult to make on the basis of electrochemical measurements. Even if the adsorption involves an ionic species O " or OH , the compensating charge on the Pt must pass through the external circuit and be equal to that on the adsorbed ionic species except for the small portion compensated by changes in the charge in the outer Helmholtz plane and diffuse layer. 

The distinction between ionic and polymeric solids is not absolute, and oxides of metals with low electropositive character (e.g., HgO) or in high oxidation states (e.g., Cr03) are better described as having polar covalent bonds. A few metals in very high oxidation states form molecular oxides (e.g., Mn207, 0s04). 

Decide whether the bonds are ionic or covalent or both. (Later we shall see that this distinction is not at all sharp, but in most of the common structures, a decision from the relative positions of the atoms in the Periodic Table is possible.) Some compounds may have both ionic and covalent bonds for example, in Na2S04, the bonds between sulfur and oxygen are considered eovaient, whereas the bonds between oxygen and sodium are ionic. All bonds in a volatile compound are generally represented as being covalent. 

The natural ionicity Jab is zero for a pure covalent bond (ca = Cb) but can achieve any value between — 1 (ca = 0 pure ionic hybrid on B) and -I-1 (cb = 0 pure ionic hybrid on A), ranging smoothly between ionic and covalent limits. Do not even think about characterizing ionic and covalent as two distinct types of bonding they are merely opposite limits of a continuum of ionicity values (0 < IjabI < 1) that exhibit no... 

Explain the important distinctions between (a) ionic and covalent bonds (b) lone-pair and bond-pair electrons (c) molecular geometry and electron-group geometry (d) bond dipole and resultant dipole moment (e) polar molecule and nonpolar molecule. 

The four rather distinct forms of chemical bonding between atoms are metallic, ionic, covalent, and dispersive (Van der Waals). All of them are sub-topics of quantum electrodynamics. That is, they are all mediated by electronic and electromagnetic forces. There are also mixed cases, as in carbides and other compounds, where both metallic and covalent bonding occur. 

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