Consider The Following Resonance Structures For The Thiocyanate Ion, SCN . [-Cent] [=C=I] [:S=C=] Calculate The Formal Charge On The Oxygen Atom, Carbon Atom And Nitrogen Atom In Each Structure, Then Determine Which Structure Is Most Stable And Which Structure Is Least Stable. (5 Pts)
The Correct Answer and Explanation is :
I cannot directly access or view external links, but I can help you analyze the resonance structures for the thiocyanate ion (SCN⁻) and calculate the formal charges on each atom in the molecule.
Let’s first consider the possible resonance structures for SCN⁻. The ion consists of sulfur (S), carbon (C), and nitrogen (N) atoms with a negative charge distributed across the molecule. The typical resonance structures include various ways of distributing the double and single bonds between these atoms, such as:
- Resonance Structure 1:
- The carbon is double-bonded to sulfur (C=S).
- The nitrogen is triple-bonded to carbon (C≡N).
- The negative charge is often placed on the nitrogen atom in this structure.
- Resonance Structure 2:
- The carbon is double-bonded to sulfur (C=S).
- The nitrogen is single-bonded to carbon (C-N), with a negative charge on nitrogen and a lone pair on sulfur.
Formal Charge Calculation:
The formal charge on an atom is calculated using the formula:
[
\text{Formal Charge} = \text{Valence Electrons} – \text{Nonbonding Electrons} – \frac{1}{2} \times \text{Bonding Electrons}
]
Resonance Structure 1:
- Sulfur (S):
- Valence electrons = 6
- Nonbonding electrons = 2
- Bonding electrons = 8 (4 electrons from double bonds to carbon and nitrogen)
- Formal charge = 6 – 2 – 4 = 0
- Carbon (C):
- Valence electrons = 4
- Nonbonding electrons = 0
- Bonding electrons = 8 (4 electrons from bonds to sulfur and nitrogen)
- Formal charge = 4 – 0 – 4 = 0
- Nitrogen (N):
- Valence electrons = 5
- Nonbonding electrons = 2
- Bonding electrons = 6 (triple bond to carbon)
- Formal charge = 5 – 2 – 3 = 0
Resonance Structure 2:
- Sulfur (S):
- Valence electrons = 6
- Nonbonding electrons = 6
- Bonding electrons = 4 (single bond to carbon)
- Formal charge = 6 – 6 – 2 = -1
- Carbon (C):
- Valence electrons = 4
- Nonbonding electrons = 0
- Bonding electrons = 6 (single bond to sulfur, double bond to nitrogen)
- Formal charge = 4 – 0 – 3 = +1
- Nitrogen (N):
- Valence electrons = 5
- Nonbonding electrons = 2
- Bonding electrons = 2 (single bond to carbon)
- Formal charge = 5 – 2 – 1 = 0
Stability Analysis:
- Most Stable Resonance Structure:
The structure where the formal charges are closest to zero is typically the most stable. In Resonance Structure 1, all the atoms (S, C, N) have a formal charge of zero, which suggests this is the most stable structure. There are no formal charges on the atoms, leading to a balanced electron distribution. - Least Stable Resonance Structure:
In Resonance Structure 2, there are formal charges on both sulfur and carbon, with sulfur carrying a negative charge and carbon carrying a positive charge. This makes this structure less stable compared to Resonance Structure 1. The presence of formal charges on adjacent atoms (like sulfur and carbon) tends to destabilize the structure.
Thus, Resonance Structure 1 is the most stable, and Resonance Structure 2 is the least stable.
Conclusion:
The most stable resonance structure is the one where formal charges are minimized, and the formal charges on each atom are close to zero. The least stable resonance structure occurs when formal charges are more unevenly distributed, as seen in the second resonance structure.