TTU General Chemistry

One student wrote:
> 
> Dr. Northrup
> 
> While studying, I realized I really didn't know the difference between
> covalent and polar bonds.  What are they and when do you expect them?  Then
> there are polar covalent bonds.  Please help.  Thanks!
> 
> xxxxxxxx

Good question. Let me see if I can clarify:

Covalent bond = a bond in which the electrons are shared by the partner atoms.

Polar bond = simply a type of covalent bond in which the electrons are
shared but not equally. This happens when two different atoms come
together, such a C and O.  Each atom has a different ability to draw
electrons  to itself when it shares electrons (that drawing ability is
called Electronegativity).

Polar covalent bond = same as a "polar bond". 

Nonpolar bond = two of the same atoms come together, such as the
diatomic molecule N2, or triatomics such as O3, etc.  A nonpolar
covalent bond could be viewed as having "pure" covalent character. There is
perfectly equal sharing. 


Polar molecule = a molecule in which the polar bonds are disposed in
such a way that it imparts an asymmetry to the molecule as a whole. That
is, it gives the whole molecule a Dipole Moment:

        example:  water   H-O-H    has two polar bonds. Since this molecule is
Bent, those two polar bonds ADD TOGETHER to produce a molecule with a
dipole moment. That is, a polar molecule.

        example:  carbon dioxide  O=C=O  has two polar bonds also.  Since this
molecule is Linear, those two polar bonds ADD TOGETHER to cancel each
other out, to produce a molecule with NO dipole moment. That is, a
nonpolar molecule. 

       example:  carbon tetrachloride  CCl4 has four very polar bonds
but these bonds are pointing to the corners of a tetrahedron, and form a very
symmetric arrangement. The polarity cancels itself out, and the molecule is
NONPOLAR. 


To determine whether a molecule is polar or not, 

1. First look at each chemical bond in the molecule and determine whether it is a polar bond or not.  A 
bond will be polar any time the two atoms in the bond have different electronegativities.  This happens 
essentially any time the two atoms are different.  For example, C-0 is polar C-C is not.  However, 
consider a C-H bond as essentially nonpolar, since the EN's are so close to each other. 

2.  Represent each polar bond as a vector in space, radiating out from the central atom.  

3.  Add the vectors together mentally.  IF the symmetry of the molecule causes these vectors to add to get a 
zero vector, you will end up with a NONPOLAR molecule.  

e.g.   O=C=O    is linear, with two polar bonds <---C--->  pointing in opposite directions.  The molecule 
                is NONPOLAR.
         S
e.g.    / \      H2S is polar, since the two vectors add to get a net dipole vector. 
       H   H

4.  Consider pure hydrocarbons to be essentially nonpolar since they are made up of basically 
nonpolar C-H bonds. 


Polarity will have very tremendous consequences as we shall see in the study
of liquids and solids (condensed phases).