Draw three possible Lewis structure for the thiocyanate ion , NCS’ Assign formal charge and label the best structure_
The Correct Answer and Explanation is:
The thiocyanate ion (NCS⁻) consists of three atoms: nitrogen (N), carbon (C), and sulfur (S). These atoms can be arranged in different ways to create multiple resonance structures. Below are the three most plausible Lewis structures for the thiocyanate ion:
Structure 1:
- The nitrogen atom is triple-bonded to the carbon atom (N≡C), and the carbon is single-bonded to sulfur (C-S).
- Nitrogen has a lone pair, carbon has one lone pair, and sulfur has two lone pairs.
- The formal charge on nitrogen is 0, on carbon is 0, and on sulfur is -1.
Formal Charges:
- Nitrogen: Formal charge = 5 (valence electrons) – 2 (lone pair electrons) – 3 (bonds) = 0
- Carbon: Formal charge = 4 (valence electrons) – 2 (lone pair electrons) – 4 (bonds) = 0
- Sulfur: Formal charge = 6 (valence electrons) – 6 (lone pair electrons) – 2 (bonds) = -1
Structure 2:
- The nitrogen atom is single-bonded to carbon (N-C), and the carbon is triple-bonded to sulfur (C≡S).
- Nitrogen has three lone pairs, carbon has one lone pair, and sulfur has one lone pair.
- The formal charge on nitrogen is -1, on carbon is 0, and on sulfur is 0.
Formal Charges:
- Nitrogen: Formal charge = 5 (valence electrons) – 6 (lone pair electrons) – 1 (bond) = -1
- Carbon: Formal charge = 4 (valence electrons) – 2 (lone pair electrons) – 4 (bonds) = 0
- Sulfur: Formal charge = 6 (valence electrons) – 2 (lone pair electrons) – 6 (bonds) = 0
Structure 3:
- The nitrogen atom is single-bonded to the carbon atom (N-C), and the carbon atom is single-bonded to sulfur (C-S).
- Nitrogen has two lone pairs, carbon has two lone pairs, and sulfur has two lone pairs.
- The formal charge on nitrogen is +1, on carbon is -1, and on sulfur is 0.
Formal Charges:
- Nitrogen: Formal charge = 5 (valence electrons) – 4 (lone pair electrons) – 1 (bond) = +1
- Carbon: Formal charge = 4 (valence electrons) – 4 (lone pair electrons) – 2 (bonds) = -1
- Sulfur: Formal charge = 6 (valence electrons) – 4 (lone pair electrons) – 2 (bonds) = 0
Best Structure:
The best Lewis structure is Structure 1, where the formal charges are closest to 0. In this structure, nitrogen and carbon are involved in a triple bond, and sulfur holds a single bond with carbon, making it a stable arrangement with the lowest possible formal charges. This structure also adheres to the octet rule more closely than the others.
Conclusion:
The best Lewis structure for the thiocyanate ion is the one with the lowest formal charges, which occurs in Structure 1. This structure minimizes electron repulsion and follows the principles of formal charge distribution most effectively.
