Percent ionic character

For example, for quadricovalent Cu3- (sp3 bonds) the charge on the copper atom becomes — 1 for 50 percent ionic character of the four bonds. This amount of ionic character for three bonds leads to Cu1-6-, for which x = 1.9 —(1.5X0.2) = 1.6. 

When the difference in electronegativities is great, the orbital may be so far over to one side that it barely covers the other nucleus. This is an ionic bond, which is seen to arise naturally out of the previous discussion, leaving us with basically only one type of bond in organic molecules. Most bonds can be considered intermediate between ionic and covalent. We speak of percent ionic character of a bond, which indicates the extent of electron-cloud distortion. There is a continuous gradation from ionic to covalent bonds. 

Electronegativity difference Type of bond Percent ionic character... 

Formula Electronegativity difference Percent ionic character Type of bond... 

The reactivity of organometallic compounds increases with the percent ionic character of the C-M bond. 

FIGURE 3.10 The variation in percent ionic character to a bond and the difference in the electronegativities of the atoms. 

Having shown that the weighting coefficient (A) of the term giving the contribution of an ionic structure to the molecular wave function is related to the dipole moment of the molecule, it is logical to expect that equations could be developed that relate the ionic character of a bond to the electronegativities of the atoms. Two such equations that give the percent ionic character of the bond in terms of the electronegativities of the atoms are... 

Although the equations look very different, the calculated values for the percent ionic character are approximately equal for many types of bonds. If the difference in electronegativity is 1.0, Eq. (3.70) predicts 19.5% ionic character while Eq. (3.71) gives a value of 18%. This difference is insignificant for most purposes. After one of these equations is used to estimate the percent ionic character, Eq. (3.61) can be used to determine the coefficient A in the molecular wave function. Figure 3.10 shows how percent ionic character varies with the difference in electronegativity. 

What Pauling electronegativity is predicted for an element X if the ff-Xbond energy is 402 kj mol-1 The ff-ff bond energy is 432 kj mol-1 and the X-X bond energy is 335 kj mol-1. What would be the percent ionic character of the H-X bond If the molecular wave function is written as... 

The percent ionic character of a bond is based on the difference in electronegativity of its constituent atoms and Figure 11.7. 

Another way to classify bonding involves calculating the percent ionic character of the bond. Search for information about this approach on the Internet, and write a summary of your findings. In particular, answer this question Can a bond ever be 100% covalent or 100% ionic To start your search, access the web site above. Click on Web Links to find out where to go next. 

You can use electronegativity differences to think of chemical bonds as having a percent ionic or a percent covalent character. The graph below plots percent ionic character versus AEN for a number of gaseous binary molecules. Use this graph to answer the questions on the next page. 

One of the reasons for the preparation of the present article were the following statements made by Kaufman, Wharton, and Klemperer which seem to require clarification SrO has the largest percent ionic character, pijer [0.97 according to Table III in Ref. 8] yet observed for any diatomic molecule (see Abstract) Yoshimine s calculation predicts [for the BeO molecule] a dipole moment of 7.29 D and 114% ionic character (p. 952). 

Percent Ionic Character Based on Difference in Electronegativity... 

According to Equation 3-15 bonds between atoms with electronegativity difference 1.7 have 50 percent ionic character and 50 percent covalent character. Thus bonds between fluorine and any of the metals or of the elements H, B, P, As, Te, with electronegativity near 2, are largely ionic in character, and bonds between oxygen and any of the metals are 50 percent or more ionic. For a molecule such as HF, containing a single bond, we have discussed the bond type in terms of... 

The Alkali Metals.—Bonds of the alkali metals with all nonmetals are essentially ionic (with more than 50 percent ionic character—electronegativity difference greater than 1.7) except for Li—I, Li—C, and Li—S, with about 43 percent ionic character. 

The Third-Group Elements.—The B—F bond has about 63 percent ionic character, B—O 44 percent, B—Cl 22 percent, and so forth. Bor,on forms normal covalent bonds with hydrogen. The aluminum bonds are similar to those of beryllium in ionic character. 

The Fourth-Group Elements.—The C—F bond, with 44 percent ionic character, is the most ionic of the bonds of carbon with nonmetallic elements. The Si—F bond has 70 percent ionic character, and Si—Cl 30 percent. The Si—O bond is of especial interest because of its importance in the silicates. It is seen to have 50 percent ionic character, the value of Xo — ssi being 1.7. 

The Remaining Nonmetallic Elements.—The bonds formed by fluorine with all of the metals are essentially ionic in character, and those with the intermediate elements (II, B, P, etc.) have a little more than 50 percent ionic character. The C-—F, S—F, and I—F bonds are expected to have 44 percent ionic character. In CF4, SF0, IF, and IF7 the amounts of ionic character of the bonds are probably somewhat less than this value because of the transfer of positive charge to the central atom, which increases its x value and decreases the ionic character of the bonds. 

Since the nonmetallic elements in each row of the periodic table are separated by intervals of 0.5, the bonds formed by a nonmetallic atom with immediate neighbors in the same row have 6 percent ionic character and those wTith its neighbors once removed 22 percent. 

With the assumption that the 4100 structures have equal weights, the carbon-carbon bonds in the ring are calculated to have bond number n = 1.173 and the nickel-carbon bonds to have n = 0.439, with 34.6 percent d character for the nickel bond orbitals. The number of unshared pairs on the nickel atom is 2.89. The formal charge on the nickel atom is —0.64 of this, the 4.39 Ni—C bonds, with 12 percent ionic character, would provide the opposite charge -f 0.53, leaving the nickel atom essentially neutral (charge —0.11). 

This large amount of electron transfer is not incompatible with the electroneutrality principle. The electronegativity of aluminum is 1.5, and that of gold is 2.4. The difference corresponds to 18 percent ionic character of the Au—A1 bonds, which with valence 6.60 for gold would lead to the charge —1.19 on the gold atom. To restore it to neutrality 1.19 electrons would have to be transferred to two aluminum atoms. 

Calculate approximate values for the bond energy and the percent ionic character for the C-Cl bond in CC14. 

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