Question 10.11: Using VSEPR Theory to Predict the Shape of a Molecule with a...
Using VSEPR Theory to Predict the Shape of a Molecule with a Multiple Covalent Bond
Predict the molecular geometry of formaldehyde, H_2 CO, used to make a number of polymers, such as melamine resins. The Lewis structure of the H_2 CO molecule is shown here.
Analyze
We see from the Lewis structure that the carbon oxygen bond is a double bond. When considering molecules with double or triple bonds, we treat a double or triple bond as a single electron group.
\begin{array}{r c}\begin{matrix} \ H\ _{_\diagdown }\quad\quad\quad _{_\diagup }H \\C \\ \overset{||}{O} \end{matrix} \end{array}
There are three electron groups around the C atom, two groups in the carbon-to-hydrogen single bonds and the third group in the carbon-to-oxygen double bond. The electron-group geometry for three electron groups is trigonal\text{-}planar. Because all the electron groups are involved in bonding, the VSEPR notation for this molecule is A \chi_3. The molecular geometry is also trigonal-planar.
Assess
Because the geometry is trigonal planar, we expect the angle between the two \begin{matrix} H-C \end{matrix} bonds to be close to 120°.