Polar Covalent Bonds and Bond Polarity

SAMPLE PROBLEM 9.4 Determining Bond Polarity from EN Values 

Problem (a) Use a polar arrow to indicate the polarity of each bond N—H, F—N, I—Cl. 

Comment In Chapter 10, you ll see that the polarity of the bonds in a molecule contributes to the overall polarity of the molecule, which is a major factor determining the magnitudes of several physical properties. 

FOLLOW-UP PROBLEM 9.4 Arrange each set of bonds in order of increasing polarity, and indicate bond polarity with 8-F and 8— symbols  

Problem (a) Use a polar arrow to indicate the polarity of each bond N—H, F—N, I—Cl. (b) Rank the following bonds in order of increasing polarity H—N, H—O, H—C. 

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The Ionic Bonding Model Importance of Lattice Energy How the Model Explains the Properties Polar Covalent Bonds and Bond Polarity... 

The unequal distribution of charge produced when elements of different electronegativities combine causes a polarity of the covalent bond joining them and, unless this polarity is balanced by an equal and opposite polarity, the molecule will be a dipole and have a dipole moment (for example, a hydrogen halide). Carbon tetrachloride is one of a relatively few examples in which a strong polarity does not result in a molecular dipole. It has a tetrahedral configuration... 

Formal charges are based on Lewis structures m which electrons are considered to be shared equally between covalently bonded atoms Actually polarization of N—H bonds m ammonium ion and of B—H bonds m borohydride leads to some transfer of positive and negative charge respectively to the hydrogens... 

One of the unshared pairs of the hydroxide oxygen is used to form a covalent bond to the positively polar ized proton of hydrogen fluoride The covalent bond betwen H and F in hydrogen fluoride breaks with the pair of electrons in this bond becoming an unshared pair of fluoride ion... 

For each of the following molecules that contain polar covalent bonds indicate the positive and negative ends of the dipole using the symbol -<- Refer to Table 1 2 as needed... 

Carbon-oxygen and carbon-halogen bonds are polar covalent bonds and carbon bears a partial positive charge in alcohols ( " C—0 ) and in alkyl halides ( " C—X ) Alcohols and alkyl halides are polar molecules The dipole moments of methanol and chloromethane are very similar to each other and to water... 

Lithium hydride reacts vigorously with siUcates above 180°C. Therefore, glass, quart2, and porcelain containers cannot be used in preparative processes. That only traces dissolve in polar solvents such as ether reflects its significant (60—75%) covalent bond character. It is completely soluble in, and forms eutectic melting compositions with, a number of fused salts. 

Another fundamental property of chemical bonds is polarity. In general, it is to be expected that the distribution of the pair of electrons in a covalent bond will favor one of the two atoms. The tendency of an atom to attract electrons is called electronegativity. There are a number of different approaches to assigning electronegativity, and most are numerically scaled to a definition originally proposed by Pauling. Part A of Table 1.6... 

The unequal distribution of electron density in covalent bonds produces a bond dipole, the magnitude of which is expressed by the dipole moment, having the units of charge times distance. Bonds with significant bond dipoles are described as being polar. The bond and group dipole moments of some typical substituents are shown in Table 1.7. 

The polarity of covalent bonds between carbon and substituents is the basis of important structure-reactivity relationships in organic chemistry. The effects of polar bonds are generally considered to be transmitted in two ways. Successive polarization through bonds is called the inductive fect. It is expected that such an effect would diminish as the number of intervening bonds increases. 

The difficulty of assigning a formal oxidation state is more acutely seen in the case of 5-coordinate NO adducts of the type [Co(NO)(salen)]. These are effectively diamagnetic and so have no unpaired electrons. They may therefore be formulated either as Co -NO or Co -NO+. The infrared absorptions ascribed to the N-O stretch lie in the range 1624-1724 cm which is at the lower end of the range said to be characteristic of NO+. But, as in all such cases which are really concerned with the differing polarities of covalent bonds, such formalism should not be taken literally. 

CHAPTER 2 Polar Covalent Bonds Acids and Bases... 

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