Putting it all together - shape and polarity

The last step in understanding why bonding matters and how it determines the behavior of molecules in the "real" world is to combine our understanding of bond polarity with our knowledge of shapes and structures.

For instance, the molecule methane (CH₄) has four hydrogen atoms around the central carbon so the structure and shape are both tetrahedron. Since carbon is more electronegative than hydrogen, each of the bonds is polar with the hydrogen end of each bond being somewhat positive and the carbon end being somewhat negative. 

 

However, when we visualize the molecule in its correct shape, we notice that everything on the outside of the molecule is positive. The negative carbon is buried inside the molecule.

Since like charges repel, if two of these molecules bump into each other they will not exhibit a strong attraction to each other. As a result, methane molecules do not stick together, they remain independent, and therefore methane is a gas at room temperature. 

With all charges on the outside of the molecule being the same, we say that methane is not polar.

Water, on the other hand, is also a tetrahedron and also has polar bonds, but the result of combining the polarity of the bonds with the shape of the molecule yields very different results. 

Each bond is polar toward the oxygen (since it is more electronegative than hydrogen) making the hydrogen atoms somewhat positive. In this case, however, two of the four things surrounding the central oxygen are lone pairs of electrons, which are negative by definition.

In a molecule of water, there are 2 things that are positive and 2 that are negative. As a result, if one water gets close to another, a positive hydrogen on the first water can attract a negative lone pair on the other and the two molecules will stick together. As a result of this attraction, water tends to clump together and is consequently a liquid at room temperature. We say that the water molecule (with both positive parts and negative parts on the outside) is polar.

A simple understanding of polarity could be stated this way: A molecule is considered polar if it has at least one thing on the outside that is positive and at least one thing on the outside that is negative.

Thus, if all of the things on the outside of a molecule are the same (either positive or negative), the molecule cannot be polar. Unfortunately, the converse is not necessarily true. The bonds in PH3 are not polar, so even though the four things on the outside of the molecule are not the same (3 hydrogen atoms and one lone pair), the molecule does not have anything positive on the outside and is therefore non-polar.

A slightly different way of looking at polarity of molecules