3.7: Polar Covalent Bonds and Electronegativity Electronegativity Electronegativity

3.7: Polar Covalent Bonds and Electronegativity Electronegativity Electronegativity

3.7: Polar Covalent Bonds and Electronegativity Electronegativity Electronegativity Electronegativity Electronegativity is is aa measure measure of of an an element element to

to attract attract electrons electrons toward toward itself itself when when bonded bonded to to another another element. element. An electronegative element attracts electrons. An electropositive element releases

electrons. Pauling PaulingElectronegativity ElectronegativityScale Scale Li Be B C N

O F 1.0 1.5 2.0 2.5 3.0

3.5 4.0 Na Mg Al Si P S

Cl 0.9 1.2 1.5 1.8 2.1 2.5

3.0 Electronegativity Electronegativity increases increases from from left left to to right right in in the the periodic periodic table. table. Electronegativity

Electronegativity decreases decreases going going down down aa group. group. Generalization Generalization The greater the difference in electronegativity between two bonded atoms; the more polar the .. .. bond. :

: HH : .. F F: .. N N nonpolar bonds connect atoms of the same electronegativity

Generalization Generalization The greater the difference in electronegativity between two bonded atoms; the more polar the d+- d++ d+d+d++ : d++ .. d++ .. d+bond. O C O .. : : H

F .. H O .. H .. polar bonds connect atoms of different electronegativity

3.7 Molecular Dipole Moments Dipole Moment A substance possesses a dipole moment if its centers of positive and negative charge do not coincide. m=exd (expressed in Debye units) + not polar Dipole Moment A substance possesses a dipole moment

if its centers of positive and negative charge do not coincide. m=exd (expressed in Debye units) + polar Molecular Dipole Moments d+- O d++

C O d+- molecule must have polar bonds necessary, but not sufficient need to know molecular shape because individual bond dipoles can cancel Molecular Dipole Moments O

C O Carbon dioxide has no dipole moment; m = 0 D Comparison of Dipole Moments Carbon tetrachloride m=0D Dichloromethane

m = 1.62 D Carbon tetrachloride Resultant of these two bond dipoles is Resultant of these two bond dipoles is m=0D Carbon tetrachloride has no dipole moment because all of the individual bond dipoles cancel. Dichloromethane

Resultant of these two bond dipoles is Resultant of these two bond dipoles is m = 1.62 D The individual bond dipoles do not cancel in dichloromethane; it has a dipole moment.

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