Electronegativity Difference & Polarity

In our periodic table unit, we learned about electronegativity.  

Quick review of electronegativity:
Definition:  the relative tendency of an atom to attract electrons during chemical bonding
Trend:  the electronegativity increases up a column and across a row on the periodic table; Fluorine has the highest electronegativity.

New Information about electronegativity:
*Attached below is a chart of electronegativity values.  You'll NOT need to memorize these numbers, but we will allow you to use the chart on tests and quizzes.

POLARITY
Definition:  oppositely charged ends found in a chemical compound  ("pole" means "opposite"; ex. North Pole and South Pole)

Relation to bonds:  polarity is based upon the DIFFERENCE in electronegativity values of the atoms that make up the compound.

SIMPLE WAY TO JUDGE POLARITY OF A MOLECULE: 
1.  Draw the Lewis Structure.
2.  Find the central atom, and look at the surrounding atoms and electron pairs.
3.  Is there a surrounding atom/pair that differs from the others making the molecule asymmetrical?  If so, then the molecule is polar.
4.  Is there a lone pair of electrons on the central atom?  If so, then the molecule is polar.
5.  Generally, only symmetrical or diatomic molecules are nonpolar.



Example One:  H2O is a polar covalent molecule.  
1.  Draw the Lewis dot structure of water to see the two bonds.  Notice that water has two identical bonds.  Both are connecting H to O.

2.  Notice that the central atom, oxygen, has two lone pairs of electrons.
3.  If a molecule contains lone pairs on the central atom, then it is a polar molecule.

Example Two:  CH4 is a nonpolar covalent molecule.
1.  Draw the Lewis dot structure of CH4 to see the four bonds.  Notice that CH4 has four identical bonds.  All are connecting C to H.

2.  Notice that the central atom does NOT have any lone pairs of electrons.
3.  Notice that the four bonds are identical, and the molecule is symmetrical.
4.  If there are NO lone pairs on the central atom and the molecule is symmetrical, then the molecule is nonpolar.


Example Three:  CCl4 is a nonpolar molecule.


Additional Resource:
Click here to visit Norton ChemTours, and select Chapter 9 Partial Charges and Bond Dipoles.