Question 25 3 pts A student attempts to draw the Lewis structure of PS2 Below is the structure he drew. When his teacher grades his exam; which of the following mistakes should she take off points for? [S=P_S] No brackets should be placed around each structure The correct skeletal structure should have been 5-P-S thc clement should have been central Thc: student failed to put lone pair electrons around the $ and The student failed t0 put negative charge outside of cach set of brackets for each structure This is a resonance structure but the student only drew one of two structures
The Correct Answer and Explanation is:
Correct Answer:
- ✅ The correct skeletal structure should have been S–P–S (the element sulfur should be central).
- ✅ The student failed to put lone pair electrons around the S and P atoms.
- ✅ The student failed to put negative charge outside of each set of brackets for each structure.
- ✅ This is a resonance structure but the student only drew one of two structures.
- ❌ No brackets should be placed around each structure – This is incorrect; brackets should be used because the molecule carries a charge.
Explanation
The correct Lewis structure of the PS₂⁻ ion must follow the standard rules for drawing polyatomic ions, including assigning charges, identifying the central atom, completing octets, and recognizing resonance when applicable.
1. Skeletal Structure and Central Atom:
In the incorrect student drawing, phosphorus (P) is placed in the center with double bonds to two sulfur atoms, which may seem chemically plausible. However, the more accurate skeletal structure should place sulfur (S) in the center, flanked by two phosphorus atoms. This is because sulfur, being larger and more electronegative than phosphorus, can better accommodate multiple bonding and lone pairs, and it often acts as a central atom in sulfide-based polyatomic ions. That said, in some literature, both S–P–S and P–S–P structures have been proposed depending on the context—but most importantly, the charge and electron count must be handled correctly.
2. Lone Pairs:
The student failed to show all the lone pair electrons on both sulfur and phosphorus atoms. Lone pairs are essential in completing the octet rule and determining the shape and reactivity of the molecule.
3. Charge and Brackets:
Since PS₂ carries a net negative charge, the Lewis structure must be enclosed in brackets, and the charge must be written outside (e.g., …structure……structure…⁻). The student neglected this, which is a key part of Lewis structures for ions.
4. Resonance:
The PS₂⁻ ion exhibits resonance, meaning more than one valid Lewis structure can be drawn by shifting the double bond between the sulfur and phosphorus atoms. The student only provided one structure, failing to represent resonance.
Each of these omissions or errors indicates a misunderstanding of Lewis structure conventions and electron distribution principles.
