Determining Molecular Polarity

Chemistry                                                                                                              Guided Instructional Activity 4-7
Unit 4: Compounds and Reactions                                                                                            Reading: Chapter 8

Based on bond polarity and the symmetry or asymmetry of the molecule molecular polarity can be determined.  For a molecule to be polar two conditions must be met.

Condition 1:  The bonds between the atoms must be polar for the molecule to be polar.  If the bonds are non-polar, go no further.  The molecule is non-polar.  i.e. in O₃ the bonds between O atoms must be non-polar since the electronegativity difference must be zero, 0.  Thus the molecule also must be non-polar.

Condition 2: If the bonds are polar then the geometric symmetry must be considered.  If the geometry is such that there exists a permanent, partial separation of charge on the molecule, the molecule is polar. 

Methods to determine whether a molecule is polar or not

Method 1: Draw the molecule as accurately as you can according to its geometry.  Label the more electronegative element(s) with a “d-“ and the less electronegative elements with a “d+“.  If there is clearly one “side” or end of the molecule that is “d-“ and one that is “d+“, then the molecule is polar.

Method 2: Draw the molecule as accurately as you can according to its geometry.  Draw arrows (+à) along each bond with the “+” end over the less electronegative element and the à over the more electronegative element.  Pretend that there are forces pulling in the direction of the arrow.  If such forces were exerted on the molecule and the molecule would move because of these forces, then the molecule is polar.  (This method is a crude way of describing vector addition.  If you understand vector addition, then use the arrows as vectors on your “accurately” drawn geometry.  If there is no net vector then the molecule is non-polar.)

Method 3: “The Ause Rule”  The Ause Rule is a simplification that must be used with care but is helpful.  Essentially, the Ause Rule states:  If the bonds are polar and there is a lone pair on the central atom then the molecule is polar.”  (This works in cases where there are 2-4 electron groups around the central atom and where all of the ligands are the same element.)

Using which ever method you would like, determine whether each molecule is polar or not for the following compounds:

(a)    H₂S

(b)   CO₂

(g)    CF₄                                              

(h)    O₂

(h)     NCl₃

(i)       SO₃

(j)      O₃