electronegative than C or S. Therefore, we expect any neg-ative formal charge to reside on the N atom (guideline 2). For these two reasons, the middle Lewis structure is the dominant one for NCS.
Practice Exercise 1
The sulfate ion, SO42, can be drawn in many ways. If you minimize formal charge on the sulfur, how many S-O
double bonds should you draw in the Lewis structure?
(a) 0 (b) 1 (c) 2 (
d) 3 (e) 4
Practice Exercise 2
The cyanate ion, NCO”, has three possible Lewis structures.
(a) Draw these three structures and assign formal charges in each. (b) Which Lewis structure is dominant
The Correct Answer and Explanation is :
Practice Exercise 1: The Sulfate Ion (SO₄²⁻)
To minimize the formal charge on the sulfur atom in the sulfate ion, we need to distribute electrons optimally. Sulfur is in group 16 and can expand its octet, so we can place double bonds to reduce formal charge. The best structure typically includes two S=O double bonds, meaning the correct answer is:
(c) 2 double bonds
Practice Exercise 2: The Cyanate Ion (NCO⁻)
(a) Possible Lewis Structures and Formal Charges
The cyanate ion (NCO⁻) can have three major resonance structures:
- Structure 1: N=C=O (with a negative charge on N)
- Formal charges:
- N: -1 (5 valence – 4 bonding – 0 lone pairs = -1)
- C: 0 (4 valence – 4 bonding = 0)
- O: 0 (6 valence – 4 bonding – 2 lone pairs = 0)
- Structure 2: N-C≡O (with a negative charge on O)
- Formal charges:
- N: 0 (5 valence – 3 bonding – 2 lone pairs = 0)
- C: 0 (4 valence – 4 bonding = 0)
- O: -1 (6 valence – 3 bonding – 4 lone pairs = -1)
- Structure 3: N≡C-O (with a negative charge on O)
- Formal charges:
- N: 0 (5 valence – 3 bonding – 2 lone pairs = 0)
- C: +1 (4 valence – 3 bonding = +1)
- O: -1 (6 valence – 7 bonding – 1 lone pair = -1)
(b) Dominant Lewis Structure and Explanation
The dominant resonance structure is Structure 1: N=C=O, where the negative charge resides on the nitrogen atom.
Why is Structure 1 the Best?
- Formal Charge Distribution:
- In Structure 1, the formal charge is placed on nitrogen (-1), which is more electronegative than carbon but less electronegative than oxygen. This is a more favorable distribution compared to placing the negative charge on oxygen or creating a formal charge separation (+1 on carbon and -1 on oxygen, as seen in Structure 3).
- Electronegativity Consideration:
- Oxygen is the most electronegative element in the molecule, and placing the negative charge on oxygen (as in Structure 2) is sometimes favorable. However, resonance stability and charge delocalization in Structure 1 make it the dominant form.
- Bond Strength and Stability:
- A C=N double bond and C=O double bond create a balanced and stable structure with strong bonds, leading to less reactivity and higher stability.
- Experimental Data and Computational Studies:
- Studies suggest that cyanate ions exhibit a resonance hybrid, but the N=C=O form is most prevalent in chemical reactions and spectroscopy.
Thus, the dominant Lewis structure for the cyanate ion (NCO⁻) is N=C=O, with a negative formal charge on nitrogen.