VSEPR geometry predicts that the bond angles in formaldehyde (CH2O), a trigonal planar molecule, will be 120° throughout all three angles. However, analysis showed that the H-C-H bond angle is slightly smaller at 116.5°, while the two H-C-O angles are slightly larger at 121.8°. Explain this difference in the predicted bond angle vs actual bond angle?
The Correct Answer and Explanation is:
The difference between the predicted bond angles and the actual measured bond angles in formaldehyde (CH₂O) arises due to the uneven distribution of electron density around the central carbon atom. According to Valence Shell Electron Pair Repulsion (VSEPR) theory, formaldehyde has a trigonal planar geometry because the carbon atom forms three regions of electron density: two single bonds with hydrogen atoms and one double bond with the oxygen atom. This arrangement leads to an ideal bond angle of 120°.
However, real molecular geometry can differ from ideal predictions due to the nature of the bonds involved. In formaldehyde, the carbon-oxygen double bond has greater electron density than the carbon-hydrogen single bonds. This increased electron density causes the bonding electrons in the C=O bond to exert stronger repulsion on the bonding electron pairs in the C–H bonds. As a result, the two H–C–O bond angles are pushed slightly wider than the ideal 120°, each measuring about 121.8°.
To accommodate this shift, the H–C–H bond angle must decrease to maintain the planar structure of the molecule. Therefore, the H–C–H bond angle reduces to about 116.5°. This slight compression is necessary to allow the other two bond angles to expand under the influence of the stronger electron repulsion from the double bond.
In summary, the discrepancy between the predicted and actual bond angles in formaldehyde results from the unequal electron repulsion caused by different bond types. The stronger repulsion of the carbon-oxygen double bond causes a redistribution of bond angles that widens the H–C–O angles and narrows the H–C–H angle, while still preserving the trigonal planar shape around the carbon atom. This example highlights how VSEPR theory provides a basic framework but actual molecular geometries are influenced by bond character and electron distribution.
