Topics Covered: molecular polarity, VSEPR theory, polarity, electron domain
Last time, we talked about bond polarity that comes from the difference in electronegativity. However, like we mentioned, bond polarity does NOT ALWAYS indicate molecular polarity. This time, we will discuss how to find out whether the molecule is polar or not— using the famous VSEPR theory.
Simply speaking, a molecule is polar if it has a polar bond and its dipoles (opposite charged ends) don’t cancel out. Generally, if a molecule has a symmetric 3-D shape, it’s likely that the dipoles will cancel out; if the dipoles cancel out, then the molecule will be nonpolar since there is no unequal distribution of electrons. To find out the molecule’s three-dimensional shape, we are going to use the VSEPR theory.
VSEPR stands for Valence Shell Electron Pair Repulsion. The main idea of this theory is that because valence shell electrons always repel each other (remember like repels like?), the electron pairs will arrange themselves to minimize repulsion effects from one another: in other words, the electrons will take the shape which makes them stay farthest from each other.
To find out the VSEPR model of a molecule, we first look at the electron domain, which is the number of electron regions that are surrounding the central atom. One thing to note here, both double bonds and lone pairs count as ONE electron domain). Based on the number of electron domains, you can determine the electron geometry of a molecule, as shown below.
Then, you look at the number of LONE PAIRS (electrons that are not bonded with other atoms) around the central atom. If a molecule has no lone pairs, the molecular geometry of the molecule will be the same as the electron geometry. But if it does have lone pairs, depending on the number, the molecule will have a different name for its molecular geometry. The names are also shown below.
Let’s look at an example. A molecule PH3 has four electron domains around its central atom, so its electron geometry is tetrahedral. Now, out of those four electron domains, one of them is a lone pair. Since it has one lone pair, its molecular geometry would be a trigonal pyramid!
Going back to our lesson of polarity, let’s look at the molecule CH4 (methane). There is a bond polarity due to a difference in electronegativity between carbon and hydrogen. However, the molecular shape of this molecule is tetrahedral (four electron domains with no lone pairs). Since the dipoles cancel out in a symmetrical shape, methane will NOT be polar even though it has a polar bond.
Finally, let’s sum up the polarity lesson with a flow chart. To determine a molecule’s polarity, first look at the bond polarity. Are the bonds polar? If so, look at the molecular shape. If the molecule is non-symmetric and the dipoles don’t cancel out, the molecule will be polar!
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