Question

Using VSEPR theory, which of these molecules does NOT have a trigonal planar shape?
A) ammonia (NH3)
B) formaldehyde (H2CO)
C) sulfur trioxide (SO3)
D) boron trifluoride (BF3)

Answer 1

Answer:

Ammonia does not have a trigonal shape.

Explanation:

According to VSEPR theory:

Molecules with 3 sigma bonds and no lone pair possess trigonal planar shape irrespective of the number of pi bonds.

• Ammonia $(NH_3)$ has 3 sigma bonds and one lone pair.Hence its shape is pyramidal due to bond pair - lone pair repulsion.
• Formaldehyde (HCHO) has 3 sigma bonds and 1 pi bond.Hence it has trigonal planar shape.

• Sulfur trioxide $(SO_3)$ has 3 sigma bonds and 3 pi bonds.
• Boron triflouride (BF_3) has 3 sigma bonds.

Hence except ammonia all other molecules given possess trigonal planar shape.