Partial ionic character of covalent bonds

Recall that when atoms with different electronegativities react to form molecules, the electrons are not shared equally. The possible result is a polar covalent bond or, in the case of a large electronegativity difference, a complete transfer of one or more electrons to form ions. The cases are summarized in Fig. 8.12. 

How well can we tell the difference between an ionic bond and a polar covalent bond The only honest answer to this question is that there are probably no totally ionic bonds between discrete pairs of atoms. The evidence for this statement comes from calculations of the percent ionic character for the bonds of various binary compounds in the gas phase. These calculations are based on comparisons of the measured dipole moments for molecules of the type X—Y with the calculated dipole moments for the completely ionic case, X Y . The percent ionic character of a bond can be defined as 

Application of this definition to various compounds (in the gas phase) gives the results shown in Fig. 8.13, where percent ionic character is plotted versus the difference in the electronegativity values of X and Y. Note from this plot that ionic character increases with electronegativity difference, as expected. However, none of the bonds reaches 100% ionic character, even though compounds with the maximum possible electronegativity differences are considered. Thus, according to this definition, no individual bonds are completely ionic. This conclusion is in contrast to the usual classification of many of these compounds (as ionic solids). All the compounds shown in Fig. 8.13 with more than 50% ionic character are normally considered to be ionic solids. Recall, however, the results in Fig. 8.13 are for the gas phase, where individual XY molecules exist. These results cannot necessarily be assumed to apply to the solid state, where the existence of ions is favored by the multiple ion interactions. 

Another complication in identifying ionic compounds is that many substances contain polyatomic ions. For example, NH4CI contains NH4+ and Cl ions, and Na2S04 

Note that the compounds with ionic character greater than 50% (red) are normally considered to be ionic compounds. 

The relationship between the ionic character of a covalent bond and the electronegativity difference of the bonded atoms. 

Since the equation for iattice energy contains the product Q1Q2, the iattice energy for a soiid with 2+ and 2 ions shouid be four times that for a soiid with 1 + and 1 - ions. That is, 

The three possible types of bonds (a) a covalent bond formed between identical F atoms (b) the polar covalent bond of HF, with both ionic and covalent components and (c) an ionic bond with no electron sharing. 

The energy required to remove two electrons from the magnesium atom (735 kJ/mol for the first and 1445 kJ/mol for the second, yielding a total of 2180 kJ/mol) is much greater than the energy required to remove one electron from a sodium atom (495 kJ/mol). 

Energy (737 kJ/mol) is required to add two electrons to the oxygen atom in the gas phase. Addition of the first electron is exothermic (-141 kJ/mol), but addition of the second electron is quite endothermic (878 kJ/mol). This latter energy must be obtained indirectly, since the (g) ion is not stable. 

---

The Partial Ionic Character of Covalent Bonds and the Relative Electronegativity of Atoms... 

The Partial Ionic Character of Covalent Bonds. Often a decision must be made as to whether a molecule is to be considered as containing an ionic bond or as containing a covalent bond. There is no question about a salt of a strong metal and a strong non-metal an ionic structure is to be written for it. Thus for lithium chloride we write... 

---