Chemistry / Unit 6 Chem 1056 worked example, 60 to 90 seconds
Assigning VSEPR Geometry from a Lewis Structure
The idea
Assigning a molecule's VSEPR geometry means counting the central atom's electron domains from its completed Lewis structure, naming the electron-pair geometry from that count, then naming the molecular geometry by identifying which domains are lone pairs.
In practice
Sulfur dioxide's Lewis structure places one lone pair, one single bond, and one double bond on the central sulfur, three electron domains in total. Three domains give a trigonal planar electron-pair geometry. Because one of those three domains is a lone pair rather than a bonded atom, the molecular geometry is named bent, and the O–S–O angle is compressed slightly below the ideal 120° by the lone pair's extra repulsion.
Where people slip
Counting a double or triple bond as more than one electron domain, instead of counting each multiple bond as a single domain regardless of its bond order, produces the wrong domain count and the wrong geometry.
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