How to find molecular geometry in AP Chemistry: an example

What is the molecular geometry of $\ce{SO2}$?

• A) Bent
• B) Trigonal planar
• C) Linear
• D) Tetrahedral

Solving this Problem

Molecular geometry is the shape that the atoms in a molecule make. In order to find the molecular geometry for any problem, start by counting electron domains and lone pairs.

An electron domain is any region of electron density (meaning anywhere where there are electrons around the central atom). This sounds complicated, but really just means the number of lone pairs plus the number of bonds. In $\ce{SO2}$, there is a sulfur atom in the center with one oxygen bonded on each side. Sulfur and oxygen both have 6 valence electrons, so there are 18 total. The correct Lewis structure uses 8 electrons on double bonds on both $\ce{S}$ to $\ce{O}$ bonds, two lone pairs on each $\ce{O}$ uses 8 more, and the remaining 2 go on $\ce{S}$ in an expanded octet. Be careful here, because a double bond still counts as only one electron domain: both pairs of electrons sit in the same region around the sulfur. That leaves $\ce{SO2}$ with two bonds and one lone pair, which is 3 electron domains.

Then, count the number of lone pairs. As already covered, there is 1 lone pair in $\ce{SO2}$.

And at this point, the rest of the problem is pure memorization. Unfortunately, molecular geometry is a part of AP Chem that you just have to memorize. Check lesson 2.7 VSEPR and Hybridization if you need a refresher or want to refer to the molecular geometry chart. Since $\ce{SO2}$ has 3 electron domains and 1 lone pair, it will have the bent molecular geometry. This means that the correct answer is choice A, bent.

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