Electronegativity differences polarize bonds

Electronegativity Differences Polarize Bonds We saw how this principle applies... 

Electronegativity Differences Polarize Bonds In Section 3.1A we learned that heterolysis of a covalent bond is aided when the bond is polarized by a difference in electronegativity of the bonded atoms. We saw how this principle apphes to the heterolysis of bonds to carbon in Section 3.4 and in explaining the strength of acids in Sections 3.8 and 3.1 IB. 

In general, if the electronegativity difference is equal to or less than 0.5 the bond is nonpolar covalent, and if the electronegativity difference between bonded atoms is 0.5-1.9 the bond is polar covalent. If the difference in electronegativities between the two atoms is 2.0 or greater, the bond is ionic. Some examples are shown below. 

Figure 3-1 illustrates the effect of electronegativity differences on bond polarity. 

Electronegativity is a semiquantitative measure of the electron-attracting power of a bonded atom. The greater the electronegativity, the more the atom attracts electrons. Covalent bonding between atoms of different electronegativity yields polar bonds that between atoms of the same electronegativity yields nonpolar bonds (Section 13.3). 

Both the degree and the direction of the polarization of a bond can be predicted by the electronegativity difference. The bonding electron pair is more likely to be found around the more electronegative atom. Carbon can be either partially plus, 6+, or... 

When the electronegativity difference between bonding atoms is between 0.5 and 2.0, the electron sharing is not so rmequal that a complete transfer of electrons takes place. Instead, there is a partial transfer of the shared electrons to the more electronegative atom. The less electronegative atom stiU retains some attraction for the shared electrons. The bond that forms when electrons are shared unequally is called a polar covalent bond. A polar covalent bond has a significant degree of ionic character. 

The polarity of a molecule arises from charge separation caused by electronegativity differences in bonds, although contributions from lone-pairs and the consequences of molecular symmetry are also important. High polarity gives strong intermolecular forces, and also provides a major contribution to the dielectric constant. 

Analyze The size of the band gap depends on the vertical and horizontal positions of the elements in the periodic table. The band gap will increase when either of the following conditions is met (1) The elements are located higher up in the periodic table, where enhanced orbital overlap leads to a larger splitting between bonding and antibonding orbitals, or (2) the horizontal separation between the elements increases, which leads to an increase in the electronegativity difference and bond polarity. 

Those bonds which are highly polarized (a large electronegativity difference between bonded atoms that should result in a relatively large dipole moment) with electron pairs tightly bound would be expected to exhibit the reverse effect (strong IR and weak Raman bands). Four examples are listed. 

The bond dipoles m Table 1 3 depend on the difference m electronegativity of the bonded atoms and on the bond distance The polarity of a C—H bond is relatively low substantially less than a C—O bond for example Don t lose sight of an even more important difference between a C—H bond and a C—O bond and that is the direction of the dipole moment In a C—H bond the electrons are drawn away from H toward C In a C—O bond electrons are drawn from C toward O As we 11 see m later chap ters the kinds of reactions that a substance undergoes can often be related to the size and direction of key bond dipoles... 

What electronegativity difference, large or small, creates a more polar bond A more covalent bond ... 

Most organic compounds are electrically neutral they have no net charge, either positive or negative. We saw in Section 2.1, however, that certain bonds within a molecule, particularly the bonds in functional groups, are polar. Bond polarity is a consequence of an unsymmetrical electron distribution in a bond and is due to the difference in electronegativity of the bonded atoms. 

The extent of polarity of a covalent bond is related to the difference in electronegativities of the bonded atoms. If this difference is large, as in HF (AEN = 1.8), the bond is strongly polar. Where the difference is small, as in H—C (AEN = 0.3), the bond is only slightly polar. 

Our work described in this section clearly illustrates the importance of the nature of the cations (size, charges, electronegativities), electronegativity differences, electronic factors, and matrix effects in the structural preferences of polar intermetallics. Interplay of these crucial factors lead to important structural adaptations and deformations. We anticipate exploratory synthesis studies along the ZintI border will further result in the discovery of novel crystal structures and unique chemical bonding descriptions. 

Any covalent bond between atoms of different elements is polar to some extent, because each element has a different effective nuclear charge. Each element has a characteristic ability to attract bonding electrons. This ability is called electronegativity and is symbolized by the Greek letter chi. When two elements have different electronegativity values, a bond between their atoms is polar, and the greater the difference (A. the more polar the bond. 

Electronegativity differences (A x) between bonded atoms provide a measure of where any particular bond lies on the continuum of bond polarities. Three fluorine-containing substances, F2, HF, and CsF, represent the range of variation. At one end of the continuum, the bonding electrons in F2 are shared equally between the two fluorine atoms (A = 4.0 - 4.0 = 0). At the other limit, CsF (A = 4.0 - 0.7 = 3.3) is an ionic compound in which electrons have been fully transferred to give Cs cations and F" anions. Most bonds,... 

Dipole moments depend on bond polarities. For example, the trend in dipole moments for the hydrogen halides follows the trend in electronegativity differences the more polar the bond (indicated by Ax), the larger the molecular polarity (indicated by the dipole moment, fi ... 

Electronegativity is a scale used to determine an atom s attraction for an electron in the bonding process. Differences in electronegativities are used to predict whether the bond is pure covalent, polar covalent, or ionic. Molecules in which the electronegativity difference is zero are considered to be pure covalent. Those molecules that exhibit an electronegativity difference of more than zero but less than 1.7 are classified as polar covalent. Ionic crystals exist in those systems that have an electronegativity difference of more than 1.7. 

Describe how electronegativity differences are used to predict whether a bond is pure covalent, polar covalent, or ionic. 

These definitions are clear, but they do not apply to the vast majority of real molecules in which the bonds are neither purely ionic nor purely covalent. Lewis recognized that a pair of electrons is generally not shared equally between two electrons because the atoms generally have different powers of attracting electrons, that is, they have different electronegativities, giving charges to both atoms. Such bonds are considered to have some covalent character and some ionic character and are known as polar bonds. 

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