Electronegativity and ionic character

Although a characteristic property of covalent bonding is its directional nature, it does not follow that a molecule that has a geometry consistent with that predicted for covalent bonds, such as planar triangular for BF3, must necessarily have covalent bonds. Close packing of anions around a central cation which has an empty valence 

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In addition to x-ray studies and dipole moments, there are other suggested ways of estimating the ionic character of bonds, but these involve more advanced considerations and none is really satisfactory. It follows then that any quantitative relationship between electronegativity and ionic character (and there are many of them that have been proposed) must be taken with reserve. 

To justify such a description within quantum mechanics we are led into a consideration of ionic and covEilent contributions to an approximate wave function. If covalent contributions are minor, the bond is said to be ionic in character, and the electrostatic model is considered to be applicable. Unfortunately, the terms ionic character and covalent character are used with various meanings. This is so, in part, because the rapid development of chemical bond theory has caused a drift of the meanings of these terms over the past two decades. Pauling s definitions, as presented in his book (1585, p. 48), no doubt represent the intent of most workers as of 1940. He concluded that there is a covalent bond between two atoms X and Y if the dissociation energy of X—is the mean of the dissociation energies of X— X and Y— Y. If the dissociation energy of X— Y exceeds this mean, the excess is attributed to additional ionic character of the bond. This criterion furnishes the basis for his scale of electronegativity, and ionic character is inter-... 

Relationship Between Electronegativity Difference and Ionic Character ... 

Several authors have attempted to correlate observed bond energies with a number of parameters (viz., ionization potential, electronegativity, ionic charge and radius) representing the covalent and ionic character of the coordinate bond. Van Uitert and Fernelius (1954) assume that the bond energy AH° can be represented by AH° = /(XV X d, X m — X d, if-o) where X u and X d are the effective electronegativities of metal and ligand... 

Table I. Electronegativities of Selected Elements and Ionic Character and Ionic Bond Energies of Si-X Bonds...

Some chemists also use average electronegativity (Xav) to divide metallic bonds from ionic and covalent ones. Metallic bonds are indicated by a low value of Xav (less than about 2) with a low value of Ax (less than about 1). However, there is only a partial correlation between electronegativity and metallic character. For example, the value of x for silicon is lower than the value for some metals, and the values of Xav and Ax for TiO (which is metallic) are similar to the values for MgO and CaO. Metallic character is best gauged from electrical conductivity. 

Chemistry has a knack of using terms such as valency, electronegativity and bonding which have a multiplicity of meanings. In its broadest sense, valency has been used to describe the ability of elements to combine with others. Russell s book provides a thorough analysis of the history of valency [15]. A chemical bond is more precisely defined as the force which holds two chemical entities together, but the definition encompasses a duality which at its extremes is based on either electrostatic (ionic) or covalent bonding and in between a variable amount of covalent and ionic character. 

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