Polar Covalent Bonds

Electronegativity - The Basis for Polar Covalent Bonds

Electronegativity is the tendency of an atom to attract electrons to itself. The larger the electronegativity value, the greater the attraction.  Recall that electronegativity increases going toward the top right corner of the periodic table and decreases toward the bottom left of the periodic table.  Fluorine has the largest electronegativity and has the greatest attraction for electrons.  Cesium on the other hand has low electronegativity of 0.7.  The following periodic table has the more electronegative shaded red.  Most of the atoms important to organic chemistry reside in the upper right of the periodic table and have some of the highest electronegativities. 

A number of factors can influence the electronegativity of an element.  For example, carbons of differing hybridization (sp, sp2, sp3) have differing electronegativities.  It is a dimensionless property because it is only a tendency.

Polar Covalent Bonds

In general chemistry, you learn about ionic and covalent bonds.  You should think of ionic and covalent bonding as two possible extremes of bonding.  At the ionic bonding extreme you have compounds like Na-Cl (sodium chloride), which behave as if the Na lost two electrons to the more electronegative Cl atom.  At the other extreme, you have diatomic molecules of the same atoms (i.e. H-H, Cl-Cl), which behave as if they are sharing their two electrons equally in their bonds.  The table below shows the MEP (Molecular Electrostatic Potential). Electrostatic potential maps, also known as electrostatic potential energy maps, or molecular electrical potential surfaces, illustrate the charge distributions of molecules.  These maps allow us to visualize variably charged regions of a molecule. Knowledge of the charge distributions can be used to determine how molecules interact with one another. Sodium chloride (NaCl) on the left clearly shows considerable charge separation.  The lack of color in the diatomic chlorine is indicative of a uniform charge distribution.  The carbon-chlorine and carbon-hydrogen bonds in methyl chloride (CH₃Cl) are considered polar covalent bonds.

Ionic	Polar Covalent	Covalent
NaCl	CH3Cl	Cl-Cl

The direction of polarity can be indicated on bonds or structures using polarization arrows.  Normally the arrow points in the direction of the most electronegative element.  Oftentimes we will place a + at the start of the arrow to indicate which atom is deficient in electrons.