Molecular Geometry and Polarity
Although we draw our molecules on a flat sheet of paper they exist in 3-dimensions. Using Lewis structures we can predict the overall geometry of our molecules and the bond angles between each atom in a molecule.
VSEPR
Chemists use the valence shell electron pair repulsion (VSEPR) model to predict the shape of different molecules. The model is based on the fact that electrons will repel one another. As a result of this repulsion, the constituent atoms in a molecule will be as far apart from one another as possible.
To see how this works, let’s take a look at methane, CH4. Typically we draw methane with a central carbon atom and four surrounding hydrogen atoms each at a 90° angle from one another. Per the VSEPR model, this is incorrect since using 3D space our molecule can increase the angle between each atom to ~109.5°.
Methane accomplishes this by pushing one of its hydrogens towards us and out of the page and one away from us into the page. Resulting in a molecule with atoms that are further spaced than its flat counterpart.
Shape and Bond Angle
In order to easily and accurately determine the shape of a molecule chemists created a VSEPR chart that allows us to find the shape of an element on the basis of its bonds and lone pairs.
To use the chart begin by counting all of the electron densities present or its hybridization this includes bonds and electron pairs. For bonds, double and triple bonds are counted as one-electron density. This number corresponds to the basic geometry of the molecule. For example in a water molecule, H2O, the central O has two bonds and two lone pairs for a total of four electron densities. Based on this the basic geometry is tetrahedral the same would also be true of methane since it has a carbon center bonded to four Hs.
However, the basic geometry of a molecule only refers to how electron densities are spaced not the atoms. To determine a molecule’s shape we need to consider the number of bonded atoms it has. In water’s case, it has two while methane has four. Therefore water would be bent while methane is tetrahedral.
Polarity
This distinction might seem pedantic, but it is crucial when determining the polarity of a molecule. In the previous lesson on bonds, polar bonds result from two atoms with differing electronegativity values. While a polar molecule must possess polar bonds the arrangement of those bonds is also important.
For example, let’s compare CCl4 to NH3 to see the impact shape has on polarity. Using the lewis structures of each molecule and the VSEPR chart both CCl4 and NH3 have a basic tetrahedral geometry. Since nitrogen only has three bonded atoms NH3‘s final shape is trigonal pyramidal while CCl4 is tetrahedral.
Now that we know the overall shape of each molecule we can draw in the polarity or dipole vectors pointing in the direction of electron movement.
If we look at the two molecules we can see that CCl4’s various vectors will all cancel leaving it with no net dipole vector. As a result, CCl4 is a non-polar molecule despite its apparent polar bonds. NH3, on the other hand, is trigonal pyramidal and while some of the polarity vectors will cancel each other partially there is still an overall resultant dipole vector. If we want to find it we can line each vector up tip to tail in 3-dimensions and draw the resulting dipole in.
While polar bonds can result in both nonpolar and polar molecules, nonpolar bonds will always result in a nonpolar molecule.