Fact: When the Lewis structure of the sulfate ion, SO42- is drawn, the central atom S expands its octet to allow for 16 valence electrons to be present in the valence shell, resulting in two of the O atoms each forming a double bond with S while the other two O atoms each forming a single bond with S. The average bond length of a single S-O bond is 157 picometers (pm), while the average bond length of a double bond between S and O is 143 pm. However, the actual length of all of the bonds between S and O is known to be 149 pm in SO42-. The H-H bond length is about 74 pm. Despite what happens to S, the shape of SO42- remains as tetrahedral. True or false: There are six (6) resonance structures for SO42-. False O True
The correct answer and explanation is:
Correct Answer: False
Explanation:
The sulfate ion (SO₄²⁻) has four resonance structures, not six. Resonance structures are different ways to represent the distribution of electrons in a molecule while keeping the atomic positions unchanged.
In the Lewis structure of sulfate, sulfur is the central atom bonded to four oxygen atoms. Sulfur has an expanded octet, meaning it can accommodate more than eight valence electrons. The formal charge minimization leads to a resonance hybrid where two of the oxygen atoms have double bonds (S=O) and two have single bonds (S-O⁻) with negative charges on them.
However, since any two oxygen atoms can be chosen to form double bonds, we can rotate the positions of the double bonds among the four oxygen atoms, leading to four equivalent resonance structures.
Why Not Six?
- Only Four Ways to Arrange the Double Bonds: There are four oxygen atoms, and each resonance structure places two double bonds and two single bonds differently while maintaining overall charge distribution.
- All S-O Bonds Are Identical in Reality: Experimental data shows that all S-O bond lengths are 149 pm, which is between the typical single (157 pm) and double bond (143 pm) lengths. This suggests delocalization of electrons, meaning no single structure fully represents the molecule. Instead, the resonance hybrid is a weighted blend of these structures.
Molecular Shape:
Despite resonance, SO₄²⁻ remains tetrahedral due to four regions of electron density around sulfur, following VSEPR (Valence Shell Electron Pair Repulsion) theory.
Thus, the correct answer is False—SO₄²⁻ has four resonance structures, not six.
Image Generation Request:
A ball-and-stick model of the sulfate ion, showing:
- Sulfur in the center,
- Four oxygen atoms symmetrically arranged,
- Delocalized electron representation,
- Tetrahedral geometry.
Here is the ball-and-stick model of the sulfate ion (SO₄²⁻), showing its tetrahedral geometry and delocalized bonding due to resonance. Let me know if you need any modifications!
