Molecular Polarity

In the previous lesson you learned about the difference between polar and nonpolar covalent bonds. Take a moment now to review that section. As you hopefully recall, polar bonds form whenever two different atoms bond covalently (except for C & H).

Because the atoms have different electronegativities, the sharing of electrons is unequal, leading to partial positive and negative charges on the two atoms. We represent this partial charges with lowercase deltas. This is a polar bond. And the key thing that makes a bond polar is the asymmetric distribution of charge.

You should already know how to examine individual bonds and tell if they are polar or not. We will now learn how to do the same analysis, but for entire molecules. For a given small molecule, we want to be able to answer this question: In this molecule as a whole, is the distribution of charge symmetric?

Symmetric Molecules

A nonpolar molecule is one which is completely symmetric. One simple way for this to occur is for the molecule to contain all atoms of the same type. In such a case, none of the bonds are polar, so the distribution of charge is totally even. Two examples of this are shown here.

Molecules can also be completely symmetric, even if they contain polar bonds. This happens if you have a linear, trigonal planar, or tetrahedral molecular geometry, and if all the outer atoms are of the same type. This occurs in the molecules shown here: CCl₄, with its tetrahedral shape, and BF₃, which is trigonal planar.

In cases like these, we say that the molecule has polar bonds, but overall is nonpolar. That is, you can draw partial positive and negative charges on the C, Cl, B, and F atoms in these molecules, because each individual bond is asymmetric (if you just look at that one bond). However, the symmetric arrangement of these polar bonds means that there are no identifiable "poles" to the molecules; they don't have a positive end and a negative end (when viewed as a whole molecule).

So, for a molecule to be non-polar, it must meet one of two conditions.

It has all non-polar bonds, like the first two examples above, or like any compound of C & H.
It has polar bonds, but they are all the same type of bond (B-F or C-Cl above) and they are arranged in a tetrahedral, trigonal planar, or linear arrangement.

Asymmetric Molecules

Any molecule that doesn't meet one of those two conditions can be considered a polar molecule. There are two main ways a molecule can be polar.

First, if a molecule has lone pairs on its central atom, it is almost certainly polar. For example, while CCl₄, above, is non-polar because of its high symmetry, NCl₃, at right, is polar because there is no fourth N-Cl bond to make everything fully symmetric.

It can't be emphasized enough that this only refers to central atom lone pairs.

Second, if a molecule contains more than two elements, it is almost certainly polar. A good example is the COS molecule shown at right. While this molecule is linear, normally a symmetric arrangement, the different electronegativities of O & S mean the electrons will not be evenly distributed, so this would be a polar molecule.

Practice

You should practice determining whether molecules are polar or non-polar by completing the practice problems in your lab workbook.

Google Sites

Report abuse