Question

Which one of the following does not have a pyramidal shape?

• A. \( {(CH}_3{)}_3 {N} \)
• B. \( {(SiH}_3{)}_3 {N} \)
• C. \( {P(CH}_3{)}_3 \)
• D. \( {P(SiH}_3{)}_3 \)

Hint:

In molecules with a central atom bonded to three substituents and a lone pair, the shape is typically pyramidal. The exception occurs when the central atom does not have a lone pair (e.g., \( {(SiH}_3{)}_3 {N} \)).

Answer 1

The Correct Option is B

Step 1: A pyramidal shape is typically associated with molecules having a central atom that is bonded to three substituents with one lone pair of electrons. This geometry results from the repulsion between bonding and nonbonding electron pairs.
Step 2: In the case of \( {(CH}_3{)}_3 {N} \) and \( {P(CH}_3{)}_3 \), the nitrogen and phosphorus atoms are bonded to three substituents and have a lone pair, resulting in a trigonal pyramidal shape.
Step 3: However, in \( {(SiH}_3{)}_3 {N} \), silicon has a larger atomic size and does not exhibit a lone pair, resulting in a flat, trigonal planar shape rather than a pyramidal shape.
Step 4: Hence, the molecule \( {(SiH}_3{)}_3 {N} \) does not have a pyramidal shape.