What is the formal charge on sulfur in the most favorable Lewis structure for the SCN- (thiocyanate) ion based on minimizing formal charge overall? -1 -2 +1 +2
The Correct Answer and Explanation is:
The correct formal charge on sulfur in the most favorable Lewis structure for the SCN- (thiocyanate) ion is 0.
Explanation:
In order to determine the formal charge, we must first draw the Lewis structure of SCN-, keeping in mind that we are aiming to minimize formal charges on all atoms. Here’s how we proceed:
- Count the total number of valence electrons:
- Sulfur (S) is in Group 16, so it has 6 valence electrons.
- Carbon (C) is in Group 14, so it has 4 valence electrons.
- Nitrogen (N) is in Group 15, so it has 5 valence electrons.
- The negative charge (1-) adds an extra electron, making the total count of valence electrons:
6+4+5+1=166 + 4 + 5 + 1 = 166+4+5+1=16 valence electrons.
- Arrange the atoms:
The most logical structure for SCN- places the carbon atom in the middle, with sulfur and nitrogen on either side. This arrangement is favored because carbon can form four bonds and sulfur and nitrogen can each form two bonds in typical bonding configurations. - Construct the Lewis structure:
- Place a single bond between sulfur and carbon, and another single bond between carbon and nitrogen.
- Distribute the remaining electrons as lone pairs around the atoms, respecting the octet rule for carbon and nitrogen. Sulfur, however, can have an expanded octet since it is in period 3.
- After distributing electrons, sulfur will have two lone pairs and a single bond with carbon. Carbon will have one lone pair and two bonds (one with sulfur and one with nitrogen). Nitrogen will have two lone pairs and one bond with carbon.
- Formal charge calculation:
The formal charge formula is: Formal charge=Valence electrons−(Non-bonding electrons+12Bonding electrons)\text{Formal charge} = \text{Valence electrons} – (\text{Non-bonding electrons} + \frac{1}{2} \text{Bonding electrons})Formal charge=Valence electrons−(Non-bonding electrons+21Bonding electrons)- For sulfur:
Valence electrons = 6,
Non-bonding electrons = 4 (2 lone pairs),
Bonding electrons = 2 (from the single bond with carbon).
Formal charge on sulfur = 6−(4+1)=06 – (4 + 1) = 06−(4+1)=0. - For carbon:
Valence electrons = 4,
Non-bonding electrons = 2 (one lone pair),
Bonding electrons = 4 (from two bonds).
Formal charge on carbon = 4−(2+2)=04 – (2 + 2) = 04−(2+2)=0. - For nitrogen:
Valence electrons = 5,
Non-bonding electrons = 4 (two lone pairs),
Bonding electrons = 2 (from the single bond with carbon).
Formal charge on nitrogen = 5−(4+1)=05 – (4 + 1) = 05−(4+1)=0.
- For sulfur:
- Conclusion:
The most favorable Lewis structure has all atoms with a formal charge of 0, meaning the formal charge on sulfur is 0 in the most favorable structure. This structure minimizes formal charges overall, which is the goal in determining the most stable structure.
