View source for Polarity of molecules

Although a molecule is made up of [[Electronegativity#Polar covalent bond|polar covalent bonds]], the molecule itself may, or may not be polar. Remember that [[Bond dipole moment|bond dipole moments]] are vector quantities. In order for the molecule to be polar, the molecule's bond dipole moments must add up to give a net dipole moment. Sometimes, the bond dipole moments cancel each other to give a zero net bond dipole moment, and a nonpolar molecule results.

==Nonpolar molecule==
In some molecules, when all the bond dipole moments are summed together, the overall dipole moment is zero. In such a case, the molecule is said to be nonpolar. 

===Polarity of carbon dioxide, CO<sub>2</sub>===
Carbon dioxide, CO<sub>2</sub>, is a linear, symmetrical and nonpolar molecule. The bonds between carbon and oxygen are double bonds.

[[File:co2.gif]]

Oxygen is more electronegative than carbon. So, each carbon-oxygen double (C=O) is polar covalent with a partial negative charge on the oxygen atom. Carbon dioxide has two polar covalent bonds and yet it is a nonpolar molecule.

[[File:co2dm.gif]]

When the two bond dipole moments in carbon dioxide add up,  there is no net dipole moment. Therefore, CO<sub>2</sub> is a nonpolar molecule.

==Polar molecule==
In some molecules, when all the dipole moments are summed up, there is a net dipole moment.

===Polarity of water, H<sub>2</sub>O===

Water, H<sub>2</sub>O, is bent-shaped with a bond angle of 104.5°. 

[[File:h2o.gif]]
 
Since oxygen is more electronegative than hydrogen, water has two polar covalent H-O bonds. Each hydrogen-oxygen bond (H-O) is polar covalent with a partial negative charge on the oxygen atom.

[[File:h2odm.gif]]

When all the bond dipole moments (Vector 1 and Vector 2) are added up in the water molecule, there is a net dipole moment. Therefore, H<sub>2</sub>O is a polar molecule.

==Summary==

In summary, to determine whether a molecule is polar or not, we need to sum up all the bond dipole moments of the molecule by adding up all its bond dipole moments (ie - vectors).

* When the molecule does not possess a net dipole moment, the molecule is nonpolar.
* When the molecule possesses a net dipole moment, the molecule is polar.

==External link==
Comparison between these compounds: 
# SO<sub>2</sub> and BF<sub>3</sub>
# CH<sub>4</sub> and NH<sub>3</sub>
# PF<sub>5</sub> and SF<sub>4</sub>
# SF<sub>6</sub> and XeF<sub>4</sub>

* <span class="pops">[http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::525::530::/sites/dl/free/0072857684/322633/Polarity_of_Molecules.swf::Polarity%20of%20Molecules Polarity of Molecules]</span>


==Content suitability==
BCIT courses:
<span class="pops">[http://www.bcit.ca/study/outlines/chem0011  CHEM 0011]</span>