The Polar Covalent Bond

While covalent bonds are bonds that result from the sharing of one or more electron(s), some atoms have a greater ability to attract electrons in a bond.  These atoms are said to be more electronegative.  The most electronegative atoms are in the upper right hand corner  of the periodic table, F, and Cl. 

Because of chlorine's greater ability to attract electrons than a carbon atom, the electrons in the covalent bond between a chlorine atom and a carbon atom are not shared equally.  The electron density is greatest near the most electronegative atom.  This creates a nonsymetric electron cloud with a slight negative charge around the chlorine atom and a slight positive charge near the more electropositive carbon atom. 

Covalent molecules that form bonds with uneven sharing of electrons are called polar molecules and the individual bonds are called polar covalent bonds .  The direction of polarity for these molecules is shown using a polarity arrow, , with the arrow pointing to the negative pole and the crossed end showing the positive pole.  The same information is sometimes conveyed with a lower case Greek delta followed by either a plus or a minus sign.  A few compounds are made up of atoms with identical, or near identical, electronegativity values like, N2.  These compounds are held together in nonpolar covalent bonds


Most compounds have bonds that are both ionic and  covalent in nature. The ionic character of a bond is determined by evaluating electronegativity difference between the two elements in the bond. If that difference is large, then the bond is ionic. An intermediate value for the electronegativity difference would lead to a classification of polar covalent. When the difference is small, the bond is nonpolar. The electrogativity difference gives the magnitude and direction of polarity across each bond since the arrow always points to the more negative end of the bond.  The vector sum of all of the bonds is used to determine the overall polarity.  This gives you the polarity of a bond, but how can you determine the polarity for a whole molecule composed of multiple bonds? The geometry of the molecule is critical.

The water molecule shown below is bent because there are two pairs of unbonded electrons located on the central oxygen atom. These unbonded electron pairs or lone pairs repel the electrons of the hydrogen atoms and force the molecule into a bent position. The carbon dioxide molecule has no unpaired electrons and so is straight.  As a result of the difference in geometry between these two molecules, carbon dioxide is nonpolar while water is quite polar.

Water is perhaps the best known of polar molecules.  The polarity of covalently bonded molecules can lead to a wide range of properties.  For example, covalent compounds that are easily dissolved in water have at least some polar bonds in them, but, nonpolar compounds, like oils, do not dissolve in water.
Quick Quiz: Which of the following compounds is polar ?

Carbon dioxide
Carbon tetrachloride
carbon monoxide

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