Two possible Lewis structures for the molecule CH3S2 are given. Determine the formal charge on each atom in both structures: Answer Bank: S: Which structure is the best Lewis structure for CH3S2? The structure with carbon as the central atom. The structure with sulfur as the central atom.
Two possible Lewis structures for the molecule CH3S2 are given. Determine the formal charge on each atom in both structures: Answer Bank: S: Which structure is the best Lewis structure for CH3S2? The structure with carbon as the central atom. The structure with sulfur as the central atom.
The Correct Answer and Explanation is:
To determine the best Lewis structure for CH₃S₂ and the formal charges on each atom in both possible structures, we must evaluate the two configurations:
Structure 1: Carbon as the central atom
This structure places the carbon atom in the center, bonded to three hydrogen atoms and two sulfur atoms (linear or slightly bent geometry around C).
Connectivity:
H₃C–S–S
(Three H atoms bonded to C, then C bonded to two S atoms.)
Formal charges:
- Carbon (C): 4 valence electrons – (0 lone pair + 4 bonds) = 4 – 4 = 0
- Each Hydrogen (H): 1 valence electron – (0 lone pairs + 1 bond) = 1 – 1 = 0
- Each Sulfur (S): 6 valence electrons – (4 lone electrons + 2 bonds) = 6 – 6 = 0
So, all atoms have a formal charge of 0.
Structure 2: Sulfur as the central atom
This structure places sulfur in the center, bonded to a methyl group (CH₃) on one side and another sulfur atom on the other.
Connectivity:
CH₃–S–S
Formal charges:
- Carbon (C): 4 – (0 lone pairs + 4 bonds) = 0
- Hydrogen (H): 1 – (1 bond) = 0
- Left Sulfur (central S): 6 – (2 bonds + 4 lone electrons) = 6 – 6 = 0
- Right Sulfur (terminal S): Also 6 – (2 bonds + 4 lone electrons) = 6 – 6 = 0
Still, all formal charges are 0.
Which is the better structure?
Although both structures have formal charges of zero, the best Lewis structure for CH₃S₂ is the one with sulfur as the central atom (Structure 2).
Why?
- Electronegativity: Carbon is less electronegative than sulfur, so placing the less electronegative atom (carbon) at the terminal position next to sulfur is less favorable.
- Bonding preferences: Sulfur can form stable S–S bonds, as seen in compounds like disulfides (R–S–S–R).
- Chemical intuition and known structures: CH₃–S–S–H is analogous to methyl disulfide compounds (common in organic chemistry), favoring sulfur as the connector.
✅ Final Answer:
- Best Lewis structure: The structure with sulfur as the central atom.
- All atoms in both structures have a formal charge of 0.
