Which of the following species is not planar?

• A. NH_3
• B. SO_3
• C. BF_3
• D. CO_3^{2-}

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the concept of molecular geometry and planarity. A molecule is planar if all its atoms lie in the same geometric plane. This often depends on the central atom's hybridization and the presence of lone pairs.
2. Step 2: Analyze each species' molecular geometry based on VSEPR theory: - \(\mathrm{NH_3}\) (ammonia) has a central nitrogen atom with three bonded hydrogen atoms and one lone pair. - \(\mathrm{SO_3}\) (sulfur trioxide) has a central sulfur atom bonded to three oxygen atoms with no lone pairs. - \(\mathrm{BF_3}\) (boron trifluoride) has a central boron atom bonded to three fluorine atoms with no lone pairs. - \(\mathrm{CO_3^{2-}}\) (carbonate ion) has a central carbon atom bonded to three oxygen atoms with resonance and no lone pairs on carbon.
3. Step 3: Determine the molecular geometry and planarity for each: - \(\mathrm{NH_3}\) has a trigonal pyramidal shape due to the lone pair, which causes the molecule to be non-planar. - \(\mathrm{SO_3}\) and \(\mathrm{BF_3}\) both have trigonal planar geometries because the central atom is bonded to three atoms with no lone pairs. - \(\mathrm{CO_3^{2-}}\) is also trigonal planar due to resonance and no lone pairs on carbon.
4. Step 4: Conclude that \(\mathrm{NH_3}\) is the only species among the options that is not planar because the lone pair on nitrogen pushes the bonded atoms out of the plane, creating a three-dimensional shape.
5. Step 5: Summarize that the presence of lone pairs on the central atom is a key factor in determining non-planarity, as seen in \(\mathrm{NH_3}\), while species with no lone pairs and symmetrical bonding tend to be planar.