The number of species from the following that have pyramidal geometry around the central atom is ______
\( \text{S}_2\text{O}_3^{2-}, \, \text{SO}_4^{2-}, \, \text{SO}_3^{2-}, \, \text{S}_2\text{O}_7^{2-} \)
\( \text{S}_2\text{O}_3^{2-}, \, \text{SO}_4^{2-}, \, \text{SO}_3^{2-}, \, \text{S}_2\text{O}_7^{2-} \)
- 4
- 3
- 1
- 2
The Correct Option is C
Approach Solution - 1
Among the given species. \( \text{S}_2\text{O}_3^{2-} \) (thiosulfate ion) has a pyramidal geometry around the central sulfur atom due to the presence of a lone pair on sulfur, resulting in a trigonal pyramidal shape.\( \text{SO}_4^{2-} \) (sulfate ion) has a tetrahedral geometry due to its symmetrical distribution of oxygen atoms around the central sulfur atom.\( \text{SO}_3^{2-} \) (sulfite ion) also adopts a trigonal pyramidal structure around sulfur, but in this context, \( \text{S}_2\text{O}_3^{2-} \) is more relevant to pyramidal geometry.\( \text{S}_2\text{O}_7^{2-} \) (disulfate ion) has a tetrahedral arrangement with respect to its central atoms, making it non-pyramidal.
Therefore, only one species, \( \text{S}_2\text{O}_3^{2-} \), exhibits pyramidal geometry.
Approach Solution -2
We are asked to find how many among the given species have pyramidal geometry around the central atom. Pyramidal geometry (like in NH₃) arises when the central atom has three bonding pairs and one lone pair of electrons (sp³ hybridization with one lone pair).
The species are:
1. \( \text{S}_2\text{O}_3^{2-} \) (thiosulfate ion)
2. \( \text{SO}_4^{2-} \) (sulfate ion)
3. \( \text{SO}_3^{2-} \) (sulfite ion)
4. \( \text{S}_2\text{O}_7^{2-} \) (pyrosulfate ion)
Step 2: Analyze each species
(1) Thiosulfate ion, \( \text{S}_2\text{O}_3^{2-} \)
- Structure: one central sulfur atom bonded to three oxygen atoms (one double-bonded, two single-bonded) and one terminal sulfur atom.
- Central sulfur has 4 regions of electron density (3 S–O bonds + 1 S–S bond).
- All are bonding pairs, no lone pairs → geometry is tetrahedral.
(2) Sulfate ion, \( \text{SO}_4^{2-} \)
- Structure: sulfur atom surrounded by four oxygen atoms through sigma bonds (sp³ hybridized).
- No lone pairs on sulfur → geometry is tetrahedral.
(3) Sulfite ion, \( \text{SO}_3^{2-} \)
- Structure: sulfur atom bonded to three oxygen atoms and has one lone pair of electrons (sp³ hybridization).
- 3 bond pairs + 1 lone pair → geometry is pyramidal.
(4) Pyrosulfate ion, \( \text{S}_2\text{O}_7^{2-} \)
- Structure: two tetrahedral \(\text{SO}_4\) units joined by one oxygen bridge.
- Each sulfur atom is tetrahedrally surrounded (no lone pairs).
- Geometry around each sulfur → tetrahedral.
Step 3: Conclusion
Only \( \text{SO}_3^{2-} \) (sulfite ion) has a pyramidal geometry due to the presence of one lone pair on sulfur.
Final answer
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