Question

The number of species having non-pyramidal shape among the following is _________.
(A) SO\(_3\)
(B) NO\(_3^-\)
(C) PCl\(_3\)
(D) CO\(_3^{2-}\)

Hint:

The key to VSEPR theory is calculating the steric number (sigma bonds + lone pairs) around the central atom. A steric number of 3 with no lone pairs (AX\(_3\)) is trigonal planar. A steric number of 4 with one lone pair (AX\(_3\)E) is trigonal pyramidal.

Answer 1

Correct Answer: 3

Step 1: Understanding the Question:
We need to determine the molecular shape of four given species using VSEPR theory and count how many of them do not have a pyramidal shape.
Step 2: Detailed Explanation:
A pyramidal shape (specifically, trigonal pyramidal) arises from a central atom with 3 bonding pairs and 1 lone pair (AX\(_3\)E\(_1\) geometry). We will analyze each species.
- (A) SO\(_3\) (Sulfur Trioxide):
- Central atom: S (Group 16, 6 valence electrons).
- Bonds to 3 O atoms. Each O forms a double bond to satisfy the octet, using all 6 of S's valence electrons.
- Steric number = (number of sigma bonds) + (number of lone pairs) = 3 + 0 = 3.
- Geometry: AX\(_3\), which is trigonal planar. This is non-pyramidal.
- (B) NO\(_3^-\) (Nitrate Ion):
- Central atom: N (Group 15, 5 valence electrons).
- Bonds to 3 O atoms, plus one extra electron for the -1 charge. Total valence electrons to distribute = 5 + 3(6) + 1 = 24.
- Lewis structure shows N with one double bond and two single bonds to oxygen, with resonance.
- Steric number = 3 (sigma bonds) + 0 (lone pairs on N) = 3.
- Geometry: AX\(_3\), which is trigonal planar. This is non-pyramidal.
- (C) PCl\(_3\) (Phosphorus Trichloride):
- Central atom: P (Group 15, 5 valence electrons).
- Bonds to 3 Cl atoms, using 3 electrons for single bonds. 2 electrons remain as a lone pair.
- Steric number = 3 (sigma bonds) + 1 (lone pair) = 4.
- Electron geometry is tetrahedral, but the molecular shape is AX\(_3\)E\(_1\), which is trigonal pyramidal. This is a pyramidal shape.
- (D) CO\(_3^{2-}\) (Carbonate Ion):
- Central atom: C (Group 14, 4 valence electrons).
- Bonds to 3 O atoms, plus two extra electrons for the -2 charge. Total valence electrons = 4 + 3(6) + 2 = 24.
- Lewis structure shows C with one double bond and two single bonds to oxygen, with resonance.
- Steric number = 3 (sigma bonds) + 0 (lone pairs on C) = 3.
- Geometry: AX\(_3\), which is trigonal planar. This is non-pyramidal.
Step 3: Final Answer:
The species with non-pyramidal shapes are SO\(_3\), NO\(_3^-\), and CO\(_3^{2-}\). Therefore, the number is 3.