Chemistry · General chemistry I · Worked example
Predict a shape with a double bond
Predict the shape of formaldehyde, H₂CO, which has a carbon–oxygen double bond, and whether it is polar.
Count the domains
Carbon has two single bonds to hydrogen, one double bond to oxygen and no lone pairs. The double bond counts as one domain, so there are three.
Name the shape
Three domains lie in a plane at about 120°, and every domain holds an atom: formaldehyde is trigonal planar.
Decide the polarity
The carbon–oxygen bond is much more polar than the carbon–hydrogen bonds, and nothing opposite it cancels it, so the molecule is polar.
Result
Trigonal planar, with angles near 120°, and polar.
Your turn
Predict the shape of HCN, H–C≡N.
Show the answer and explanation
Linear.
Carbon has one single bond and one triple bond and no lone pairs: two domains, on opposite sides, so the molecule is linear with a 180° angle.
Keep exploring
In VSEPR & molecular polarity, make the center boron with three fluorines and no lone pairs. BF₃ is trigonal planar too, but its three identical bond dipoles cancel, so it is nonpolar.
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