How many resonance structures are possible for the sulfate ion; SO42 which the formal charge on sulfur (S) is zero?
The Correct Answer and Explanation is:
The number of resonance structures possible for the sulfate ion (SO₄²⁻) in which the formal charge on sulfur is zero is zero.
Explanation
The sulfate ion (SO₄²⁻) consists of a sulfur atom surrounded by four oxygen atoms, with an overall charge of -2. To explore its resonance structures, it’s essential to understand how formal charges and bonding patterns interact within the molecule.
First, consider the total number of valence electrons. Sulfur has six valence electrons, and each oxygen also has six. Therefore, the total number of valence electrons is:
6 (from S) + 4 × 6 (from O) + 2 (for the -2 charge) = 32 valence electrons.
A common way to draw SO₄²⁻ involves sulfur in the center forming four S–O bonds with the surrounding oxygens. Some of these bonds are typically represented as double bonds to minimize formal charges and complete octets. However, the question specifies that sulfur must have a formal charge of zero.
To calculate the formal charge on sulfur, we use the formula:
Formal charge = valence electrons – (nonbonding electrons + ½ bonding electrons)
If sulfur forms four single bonds with the four oxygen atoms, sulfur would have:
- 0 nonbonding electrons
- 8 bonding electrons (from four single bonds)
So the formal charge would be:
6 – (0 + 8/2) = 6 – 4 = +2
To reduce sulfur’s formal charge to zero, it would need to form double bonds, increasing its bonding electrons to 12, resulting in:
6 – (0 + 12/2) = 6 – 6 = 0
However, this leads to sulfur having an expanded octet (more than 8 electrons). While sulfur can do this because it is in Period 3, the presence of S=O double bonds reduces the negative formal charges on the oxygen atoms, not on sulfur.
When all formal charges are minimized and sulfur’s formal charge is zero, the structure is not possible without violating the octet rule for the oxygens or misallocating charges.
Thus, there are zero valid resonance structures for SO₄²⁻ in which sulfur has a formal charge of zero.
