polarity and intermolecular forces

To continue with molecular shapes, topics of Bond and Molecular Polarity  are today's focus:

Objectives:

1. Explain the role and location of electrons in a covalent bond; distinguish between nonpolar and polar covalent bonds based on electronegativity differences. 
   
   
2. Determine the polarity of molecules based on bond type and molecular shapes; relate the polarity and shape of molecules to the properties of a substance.
   
   

A covalent bond occurs when the electronegativity difference is 0.0 - 2.0.  An electronegativity difference with a range of 0.0 to 2.0 is considered nonpolar (neutral).  An electronegativity difference of 0.4 to 2.0 is considered to be polar, having a positively and a negatively charged end.  An electronegativity difference above 2.0 is a high enough separation of charge that the bond is considered ionic, with positive and negative ions attacted to eachother.

Below is an diagram with this information.  In this case, the nonpolar bond is called "pure" .

Here is 10 minute video providing an overview of the types of bonds and electronegativity difference:

The narrator in this video mentions the sharing of electrons in the covalent bond as a "tug of war" -- I've also used this idea, and I've actually used jump ropes to demonstrative covalent bonds in class.  

Jump ropes are inexpensive to purchase at your local store in the toy department.  I put a folded piece of paper with electrons drawn on them, and had students hold the end while I talked and moved the folded paper while talking about polar and nonpolar covalent bonds.


Some older sources classify the limit of the polar molecular bond electronegativity difference as 1.7 rather than 2.0.  There are some examples of bonds that don't fit the 1.7 guideline -- recognizing if there is a metal/nonmetal combination or a nonmetal/nonmetal combination is also important to establishing the type of bond.

Here's a worksheet on electronegativity differences and type of bond:

http://drkschemistry.wikispaces.com/file/view/Electronegativity%20WS1.pdf/413406098/Electronegativity%20WS1.pdf

Here's another worksheet which defines the ionic bond with an EN difference of 2.0 or higher:

http://www.chsdarkmatter.com/electronegativity.pdf

When more than one bond is combined in a molecule, the types of bonds and the symmetry of the molecular shape determines whether the entire molecule is polar or not.

Here's Mr. Post again, in a 14 minute video, to talk about the polarity of entire molecules.  Right at the beginning he points out that he's looking at the polarity of the entire molecule, not just the individual bonds.

In differentiating polar and nonpolar molecules, the physical properties of

 the substances are different as well.   Polar molecules have electric attraction to each other, so those molecules tend to stay together -- the boiling points are not necessarily high, but they tend to be liquids at room temperature.  Some examples are H2O (water), or C2H5OH (ethanol).

Nonpolar molecules have very little attraction to each other and tend to be gases at room temperature.    Many of the diatomic elements are good examples -- hydrogen, oxygen, nitrogen.  Iodine is a solid that turns directly into a gas, also known as sublimation.

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Check out my lab book "Chemistry on a Budget" at amazon.com:
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Each lab is presented with two possible report formats -- both with the same procedure -- one with 10 questions to be answered as a conclusion, the other with a full laboratory report required.  This was to give the teacher the option of what type of report is desired! 

Have a good end of the week!