Question

Draw two resonance structures for the sulfite ion \(\left(\mathrm{SO}_{3}^{2-}\right),\) one that obeys the octet rule and one in which the formal charge on the central atom is zero.

Short answer

Two resonance structures are one with sulfur forming double bonds with one oxygen (minimizing sulfur's charge) and one with only single bonds (all oxygens fully negative).

Step-by-step solution

Understand the Structure of SO3^{2-}

The sulfite ion, \( \text{SO}_3^{2-} \), consists of a sulfur atom bonded to three oxygen atoms. To draw resonance structures, we need to consider how electrons are distributed among these atoms. The sulfite ion carries an overall -2 charge.

Draw the Lewis Structure Obeying the Octet Rule

Begin by drawing sulfur (S) at the center with three single bonds to oxygen (O) atoms, assigning a lone pair on each oxygen to complete its octet. Add two additional lone pairs to the sulfur atom, making sure each oxygen and sulfur has access to 8 electrons in its valence shell. Assign the -2 charge across the structure as extra electron pairs on oxygen atoms.

Calculate Formal Charges

For each atom, calculate the formal charge: \( \text{Formal Charge} = \text{Valence Electrons} - \text{Non-bonding Electrons} - \frac{1}{2} \times (\text{Bonding Electrons}) \). For sulfur, it typically has 6 valence electrons, and each oxygen has 6 as well. Check that the total formal charge equals the ion's charge (-2).

Draw the Resonance Structure Minimizing Formal Charge on S

To minimize the formal charge on the sulfur atom, form a double bond between sulfur and one oxygen atom, while leaving the remaining oxygens with single bonds. This results in sulfur having a formal charge of 0. Redistribute electrons to ensure all atoms still satisfy the octet rule, if possible.

Verify Resonance Structures

Check that both resonance structures maintain the overall -2 charge of the ion and that the structures are valid by ensuring each atom follows the octet rule (where applicable) and the formal charge distribution is accurate.

Key concepts

Lewis structure

A Lewis structure is a diagrammatic representation of molecules that shows the arrangement of atoms and their electrons. For a molecule like the sulfite ion, SO₃²⁻, understanding Lewis structures is crucial to depict how the atoms bond and how electrons are shared or transferred. To begin drawing a Lewis structure, you:

• Identify the total number of valence electrons for the molecule. For SO₃²⁻, you need to consider the extra electrons that account for the negative charge.
• Place the least electronegative atom, sulfur in this case, as the central atom.
• Connect each oxygen atom to the sulfur using single bonds initially, distributing the remaining electrons to satisfy the octet rule where possible.

Lewis structures are valuable for visualizing molecular geometry and understanding the distribution of electrons within a molecule.

Octet rule

The octet rule is a guideline that atoms tend to form bonds until they are surrounded by eight valence electrons, mirroring the electron configuration of a noble gas. In drawing Lewis structures for molecules like the sulfite ion, the octet rule is applied to each atom, ideally:

• Oxygen atoms need to have 8 electrons around them, achieved by bonding and lone pairs.
• Sulfur can expand its octet to hold more than eight electrons, as it is in the third period of the periodic table.

However, when applying the octet rule, exceptions are sometimes made, such as for molecules possessing an odd number of electrons or central atoms that require more electrons to minimize formal charges.

Formal charge

Formal charge helps to determine the efficiency and stability of potential resonance structures by calculating the theoretical charge on each atom within a molecule.

• The formula for calculating formal charge is: \[\text{Formal Charge} = \text{Valence Electrons} - \text{Non-bonding Electrons} - \frac{1}{2} \times (\text{Bonding Electrons})\]
• For accurate resonance structures, the sum of all formal charges should equal the molecule's charge. In the case of SO₃²⁻, it totals -2.

Minimizing formal charges on each atom, especially the central atom, usually leads to more stable and favorable molecular structures. This is why creating a double bond in one of the resonance structures of SO₃²⁻ is helpful.

Sulfite ion (SO3)

The sulfite ion, represented as SO₃²⁻, consists of one sulfur atom covalently bonded to three oxygen atoms. Understanding its structure through Lewis diagrams and resonance structures is fundamental owing to its negative charge distributed among oxygen atoms.

• Sulfite is a common polyatomic ion that plays a role in many chemical processes, including sulfur chemistry in biological systems and industrial applications.
• It is important for students to visualize how such ions can form stable molecular structures via resonance, depicting electron delocalization.

The sulfite ion's study also helps in grasping key concepts like how charge distributions affect molecular stability and reactivity.

Valence electrons

Valence electrons are the outermost electrons of an atom that participate in chemical bonding. They determine how atoms interact and form compounds. In the sulfite ion

• Each sulfur atom starts with 6 valence electrons, as does each oxygen atom.
• Considering the extra two electrons giving it a -2 charge, the total count for SO₃²⁻ becomes 26 valence electrons to be considered in the structure.

The proper accounting of valence electrons is critical in Lewis structures, as it ensures each atom achieves the ideal octet configuration where feasible. Understanding the role of valence electrons further refines knowledge on how chemical bonds facilitate molecule formation.