Draw the Lewis structure for a sulfate ion, SO
. b. Determine the formal charge on each atom. c. Does this ion exhibit resonance? If so, draw the contributing structures.
The Correct Answer and Explanation is:
The sulfate ion, SO₄²⁻, is a polyatomic ion with sulfur as the central atom. The most stable Lewis structure for this ion is one that minimizes the formal charges on its atoms, which requires sulfur to have an expanded octet.
Lewis Structure and Formal Charges
The most accepted Lewis structure for the sulfate ion features a central sulfur atom connected to four oxygen atoms. Two of the oxygen atoms are connected to the sulfur by double bonds, and the other two are connected by single bonds.
- The sulfur atom has no lone pairs.
- Each of the two double-bonded oxygen atoms has two lone pairs of electrons.
- Each of the two single-bonded oxygen atoms has three lone pairs of electrons.
The overall structure is enclosed in brackets with a 2- charge to indicate it is an ion.
To determine the formal charge on each atom, we use the formula:
Formal Charge = (Valence Electrons) minus (Nonbonding Electrons) minus (1/2 * Bonding Electrons)
- Formal Charge on Sulfur (S):
Sulfur is in Group 16, so it has 6 valence electrons. In this structure, it has 0 nonbonding electrons and participates in 6 bonds (12 bonding electrons).
Formal Charge = 6 – 0 – (1/2 * 12) = 0 - Formal Charge on a Double-Bonded Oxygen (O):
Oxygen is in Group 16, so it has 6 valence electrons. It has 4 nonbonding electrons (two lone pairs) and 2 bonds (4 bonding electrons).
Formal Charge = 6 – 4 – (1/2 * 4) = 0 - Formal Charge on a Single-Bonded Oxygen (O):
It has 6 nonbonding electrons (three lone pairs) and 1 bond (2 bonding electrons).
Formal Charge = 6 – 6 – (1/2 * 2) = -1
The sum of the formal charges (0 + 0 + 0 + (-1) + (-1)) equals -2, which matches the ion’s overall charge.
Resonance Structures
Yes, the sulfate ion exhibits resonance. The placement of the two double bonds and two single bonds is not fixed. The double bonds can exist between the sulfur atom and any two of the four oxygen atoms. This results in six equivalent contributing resonance structures. The actual structure of the sulfate ion is a resonance hybrid of these six forms. In this hybrid structure, the negative charge and double bond character are delocalized, or spread evenly, across all four sulfur-oxygen bonds. This means each S-O bond is identical, with a bond order of approximately 1.5, and each oxygen atom carries an equal partial negative charge of -0.5.
