What Is the Molecular Geometry of SO₃²⁻? Complete Guide
Quick Answer: The molecular geometry of SO₃²⁻ (sulfite ion) is trigonal pyramidal with sp³ hybridization. The central sulfur atom has four electron domains (three bonding pairs and one lone pair), giving tetrahedral electron geometry but trigonal pyramidal molecular geometry. Bond angles are approximately 107.5°, and the molecule is polar due to its asymmetrical shape and uneven electron distribution.
If you’re taking General Chemistry and struggling with VSEPR theory, Lewis structures, or molecular geometry—you’re not alone. The sulfite ion (SO₃²⁻) is a classic problem that appears frequently on ALEKS assignments, MasteringChemistry modules, and Cengage MindTap quizzes.
This comprehensive guide breaks down everything you need to know about SO₃²⁻: how to draw the Lewis structure step-by-step, determine electron and molecular geometries, identify hybridization, assess polarity, and avoid the most common mistakes students make.
⚠️ Struggling with Chemistry Homework?
We complete ALEKS, MasteringChemistry, and MindTap assignments with guaranteed high grades. Whether you’re stuck on molecular geometry, Lewis structures, or any other chemistry topic—Finish My Math Class can help.
📚 Table of Contents
Step-by-Step: Drawing the Lewis Structure of SO₃²⁻
Lewis structures are the foundation for determining molecular geometry. Here’s the exact process for SO₃²⁻:
Step 1: Count Total Valence Electrons
- Sulfur (S): 6 valence electrons (Group 16)
- Oxygen (O) × 3: 6 × 3 = 18 valence electrons (Group 16)
- Negative charge (-2): Add 2 electrons
- Total: 6 + 18 + 2 = 26 valence electrons
Step 2: Determine the Central Atom
Sulfur is less electronegative than oxygen, so sulfur becomes the central atom. Arrange the three oxygen atoms around sulfur.
Step 3: Connect Atoms with Single Bonds
Draw single bonds between sulfur and each oxygen atom. This uses 6 electrons (3 bonds × 2 electrons each), leaving 20 electrons remaining.
Step 4: Complete Octets for Outer Atoms
Place 6 electrons (3 lone pairs) around each oxygen atom to complete their octets. This uses 18 electrons, leaving 2 electrons remaining.
Step 5: Place Remaining Electrons on Central Atom
Place the remaining 2 electrons as a lone pair on the central sulfur atom. Sulfur now has 8 electrons around it (3 bonding pairs + 1 lone pair).
Step 6: Check Formal Charges
Calculate formal charges to verify the structure:
- Sulfur: Formal charge = 6 – (2 + 6) = -1
- Each Oxygen: Formal charge = 6 – (6 + 2) = -1
- Total charge: -1 + (-1 × 3) = -4… Wait, that’s wrong!
Actually, the formal charge calculation shows this isn’t the optimal structure. The actual structure involves resonance with double bonds.
Corrected Structure with Resonance
The most accurate representation involves three resonance structures where one S=O double bond rotates among the three oxygen positions. In each resonance structure:
- One S=O double bond
- Two S–O single bonds
- One lone pair on sulfur
- The -2 charge is delocalized across the oxygen atoms
This resonance stabilization is important for understanding the molecule’s properties, but for VSEPR purposes, we focus on one resonance structure showing the basic bonding pattern: three S–O bonds and one lone pair on sulfur.
💡 Quick Lewis Structure Tip
For SO₃²⁻ on exams, you can draw any one of the three resonance structures—they’re all correct. The key is showing three bonding domains and one lone pair on the central sulfur atom. Most platforms accept any valid resonance structure.
Understanding VSEPR Theory
VSEPR (Valence Shell Electron Pair Repulsion) theory predicts molecular geometry based on electron domain repulsion. According to LibreTexts Chemistry, electron domains arrange themselves to minimize repulsion.
Key VSEPR Principles:
- Electron domains include bonding pairs (single, double, or triple bonds count as one domain) and lone pairs
- Electron domains arrange themselves to maximize distance between them
- Lone pairs occupy more space than bonding pairs, creating stronger repulsion
- Molecular geometry describes only the positions of atoms, not lone pairs
For SO₃²⁻: Four electron domains (3 bonding + 1 lone pair) → Tetrahedral electron geometry
Electron Geometry vs. Molecular Geometry
This distinction trips up many students—it’s critical to understand the difference:
Electron Geometry (Electron Domain Geometry)
Considers ALL electron domains—both bonding pairs and lone pairs.
For SO₃²⁻: 3 bonding pairs + 1 lone pair = 4 electron domains → Tetrahedral electron geometry
Molecular Geometry (Molecular Shape)
Considers ONLY bonded atoms—ignores lone pairs in the shape description.
For SO₃²⁻: 3 bonded atoms + 1 lone pair → Trigonal pyramidal molecular geometry
Why Bond Angles Decrease with Lone Pairs
Ideal tetrahedral angle: 109.5°
SO₃²⁻ actual angle: ~107.5°
Reason: Lone pairs occupy more space than bonding pairs because they’re only attracted to one nucleus instead of two. This extra repulsion pushes bonding pairs closer together, compressing the bond angles slightly below the ideal tetrahedral angle.
✅ Memory Trick
Electron geometry = Think electrons (all of them)
Molecular geometry = Think molecules (atoms only)
For SO₃²⁻: Electron geometry is tetrahedral (4 electron domains), but molecular geometry is trigonal pyramidal (3 atoms bonded, 1 lone pair hidden).
Hybridization of SO₃²⁻
Hybridization explains how atomic orbitals mix to form new hybrid orbitals for bonding.
Determining Hybridization from Electron Domains
For SO₃²⁻: 4 electron domains → sp³ hybridization
What This Means
The sulfur atom’s one 3s orbital and three 3p orbitals mix to form four sp³ hybrid orbitals:
- Three sp³ orbitals form sigma (σ) bonds with oxygen atoms
- One sp³ orbital holds the lone pair
This sp³ hybridization creates the tetrahedral arrangement of electron domains, even though the molecular shape is trigonal pyramidal.
Polarity Analysis: Bond Polarity vs. Molecular Polarity
Understanding polarity requires distinguishing between bond polarity and molecular polarity—a common source of confusion.
Bond Polarity
Individual S–O bonds are polar because:
- Oxygen electronegativity: 3.44
- Sulfur electronegativity: 2.58
- Difference: 0.86 (polar covalent)
Oxygen pulls electron density toward itself, creating partial negative charge (δ-) on oxygen and partial positive charge (δ+) on sulfur for each bond.
Molecular Polarity
SO₃²⁻ is polar overall because:
- Asymmetrical shape: Trigonal pyramidal geometry means bond dipoles don’t cancel
- Lone pair: Creates an asymmetric electron distribution
- Net dipole moment: Points from the sulfur (slightly positive) toward the oxygen atoms (negative)
Why Shape Matters for Polarity
Compare SO₃²⁻ (trigonal pyramidal, polar) to SO₃ (trigonal planar, nonpolar):
- SO₃: Symmetrical flat triangle → bond dipoles cancel → nonpolar
- SO₃²⁻: Asymmetrical pyramid → bond dipoles don’t cancel → polar
💡 Polarity Quick Check
If a molecule is symmetrical and all bonds are identical, it’s nonpolar (bond dipoles cancel). If it’s asymmetrical or has lone pairs on the central atom, it’s usually polar (bond dipoles don’t cancel). SO₃²⁻ has both asymmetry and a lone pair → definitely polar.
SO₃²⁻ vs. Similar Molecules: Key Differences
Understanding SO₃²⁻ in context helps cement your understanding. Here’s how it compares to similar sulfur-oxygen species:
Key Insights from Comparison
SO₃ vs. SO₃²⁻: The -2 charge adds 2 electrons, creating a lone pair. This changes geometry from trigonal planar (flat, nonpolar) to trigonal pyramidal (3D pyramid, polar).
SO₃²⁻ vs. SO₄²⁻: Sulfate has one more oxygen (4 total) and no lone pairs. Its tetrahedral geometry is perfectly symmetrical → nonpolar despite polar S–O bonds.
SO₂ vs. SO₃²⁻: Both have lone pairs and are polar, but SO₂ is bent (3 electron domains) while SO₃²⁻ is trigonal pyramidal (4 electron domains).
Common Student Mistakes with SO₃²⁻
Avoid these frequent errors that cost students points on exams and homework:
⚠️ #1 Mistake: Confusing Electron Geometry with Molecular Geometry
SO₃²⁻ has tetrahedral electron geometry (considering all 4 electron domains) but trigonal pyramidal molecular geometry (considering only the 3 bonded atoms). Always specify which geometry you’re describing. Most exam questions ask for molecular geometry unless otherwise stated.
Platform-Specific Tips: ALEKS, MasteringChemistry & MindTap
Different chemistry platforms have different quirks when it comes to SO₃²⁻ questions:
🔵 ALEKS Chemistry
Common Question Types:
- Drawing Lewis structures with the structure editor
- Selecting molecular geometry from dropdown menus
- Entering bond angles numerically
ALEKS Tips:
- The structure editor allows you to draw any valid resonance structure—don’t overthink which one to draw
- For molecular geometry, scroll through ALL options before selecting (trigonal planar and trigonal pyramidal are often next to each other)
- Bond angles: Enter 107 or 107.5 (both accepted). Don’t enter 109.5 (that’s tetrahedral)
- Polarity questions: “Polar” is correct, not “Nonpolar”
🔴 Pearson MasteringChemistry
Common Question Types:
- Multiple-choice molecular geometry questions
- Part-by-part questions (Lewis structure → geometry → polarity)
- Ranking exercises (comparing bond angles across molecules)
MasteringChemistry Tips:
- Pay attention to whether the question asks for “electron geometry” or “molecular geometry”—they’re different answers
- Some questions ask “What is the shape around the central atom?”—this means molecular geometry (trigonal pyramidal)
- Hybridization is almost always sp³, not sp²
- Watch for comparison questions: “Is the bond angle in SO₃²⁻ greater than, less than, or equal to 109.5°?” Answer: Less than (due to lone pair repulsion)
🟢 Cengage MindTap
Common Question Types:
- Interactive 3D structure visualization
- Fill-in-the-blank molecular geometry
- True/False polarity questions
MindTap Tips:
- Spelling matters: “Trigonal pyramidal” not “trigonal pyramid” or “pyramidal”
- Capitalization usually doesn’t matter, but be consistent
- 3D structure questions: rotate the molecule to see the pyramid shape clearly
- For polarity: Type “polar” or “yes” depending on question format
✅ Universal Platform Advice
No matter which platform you’re using, double-check these three things before submitting SO₃²⁻ answers:
- Did I count 26 total valence electrons (including the -2 charge)?
- Is my molecular geometry “trigonal pyramidal” (not trigonal planar)?
- Did I mark it as polar (not nonpolar)?
These three errors account for 80% of wrong answers on SO₃²⁻ problems.
Frequently Asked Questions
What is the molecular geometry of SO₃²⁻?
The molecular geometry of SO₃²⁻ (sulfite ion) is trigonal pyramidal. The central sulfur atom has four electron domains (three bonding pairs and one lone pair), giving tetrahedral electron geometry, but the molecular shape considering only bonded atoms is trigonal pyramidal.
Is SO₃²⁻ trigonal planar or trigonal pyramidal?
SO₃²⁻ is trigonal pyramidal, not trigonal planar. Trigonal planar geometry occurs when there are three bonding groups and zero lone pairs (like SO₃). SO₃²⁻ has one lone pair on the central sulfur atom, which creates a 3D pyramid shape rather than a flat triangle.
What is the bond angle in SO₃²⁻?
The bond angle in SO₃²⁻ is approximately 107.5°, slightly less than the ideal tetrahedral angle of 109.5°. The compression occurs because the lone pair on sulfur occupies more space than bonding pairs, pushing the oxygen atoms closer together.
What is the hybridization of SO₃²⁻?
The central sulfur atom in SO₃²⁻ uses sp³ hybridization. Four electron domains (three bonds + one lone pair) require four sp³ hybrid orbitals—three for bonding with oxygen atoms and one to hold the lone pair.
Is SO₃²⁻ polar or nonpolar?
SO₃²⁻ is polar. The trigonal pyramidal shape is asymmetrical, and the oxygen atoms are more electronegative than sulfur, creating an uneven distribution of electron density. The bond dipoles don’t cancel, resulting in a net dipole moment.
How many resonance structures does SO₃²⁻ have?
SO₃²⁻ has three equivalent resonance structures. In each structure, one S=O double bond rotates among the three oxygen positions while the other two remain single bonds. The negative charge is delocalized across all three oxygen atoms through resonance.
What is the electron geometry of SO₃²⁻?
The electron geometry of SO₃²⁻ is tetrahedral. This considers all four electron domains around the central sulfur atom (three bonding pairs and one lone pair). The molecular geometry is trigonal pyramidal because molecular geometry only counts bonded atoms.
How do you draw the Lewis structure of SO₃²⁻?
To draw SO₃²⁻: (1) Count 26 total valence electrons (6 from S + 18 from 3 O + 2 from -2 charge), (2) Place S in center with 3 O around it, (3) Draw single bonds (6 electrons), (4) Complete octets on O atoms (18 electrons), (5) Place remaining 2 electrons as lone pair on S, (6) Show resonance with double bond rotating among O positions.
What’s the difference between SO₃ and SO₃²⁻?
SO₃ (sulfur trioxide) is neutral with 24 valence electrons, zero lone pairs on sulfur, trigonal planar geometry, and is nonpolar. SO₃²⁻ (sulfite ion) has a -2 charge with 26 valence electrons, one lone pair on sulfur, trigonal pyramidal geometry, and is polar. The extra electrons create the lone pair that changes everything.
Why is SO₃²⁻ polar but SO₃ is nonpolar?
SO₃ is symmetrical (trigonal planar) so its bond dipoles cancel out, making it nonpolar. SO₃²⁻ is asymmetrical (trigonal pyramidal) due to the lone pair on sulfur, so bond dipoles don’t cancel, making it polar. Shape determines polarity for molecules with identical bonds.
How many lone pairs are in SO₃²⁻?
SO₃²⁻ has one lone pair on the central sulfur atom. Each oxygen atom has three lone pairs, but when determining molecular geometry, we only count lone pairs on the central atom. The one lone pair on sulfur is what makes the shape trigonal pyramidal instead of trigonal planar.
Is SO₃²⁻ the same as sulfite?
Yes. SO₃²⁻ is the chemical formula for the sulfite ion. In compounds, it appears as sodium sulfite (Na₂SO₃), calcium sulfite (CaSO₃), etc. Sulfite is used as a food preservative and appears in wine, dried fruits, and some medications.
What is the formal charge on sulfur in SO₃²⁻?
In the Lewis structure with three single bonds, sulfur has a formal charge of +1. However, with resonance structures showing one double bond, the formal charge on sulfur is 0 in the most stable resonance contributors. The -2 charge is distributed across the oxygen atoms.
Does SO₃²⁻ violate the octet rule?
No. In the basic Lewis structure, sulfur has 8 electrons around it (3 bonding pairs + 1 lone pair = 8 electrons), satisfying the octet rule. Each oxygen also has 8 electrons. Sulfur can expand its octet in some molecules, but it doesn’t need to in SO₃²⁻.
Can you do my SO₃²⁻ assignment on ALEKS?
Yes! Whether it’s ALEKS, MasteringChemistry, or Cengage MindTap, we complete chemistry assignments with guaranteed high grades. Our experts handle Lewis structures, VSEPR theory, molecular geometry, and all general chemistry topics. Get a quote for your specific assignment.
Why is the bond angle in SO₃²⁻ less than 109.5°?
Lone pairs repel more strongly than bonding pairs because they’re only attracted to one nucleus (sulfur) instead of two. This extra repulsion pushes the S–O bonding pairs closer together, compressing the bond angle from the ideal tetrahedral 109.5° to approximately 107.5°.
Is SO₃²⁻ an acid or a base?
SO₃²⁻ is a weak base. In water, sulfite ion accepts a proton (H⁺) to form HSO₃⁻ (bisulfite ion). The reaction: SO₃²⁻ + H₂O ⇌ HSO₃⁻ + OH⁻. This acid-base behavior is separate from its molecular geometry properties.
How is SO₃²⁻ different from SO₄²⁻?
SO₄²⁻ (sulfate) has four oxygen atoms and zero lone pairs on sulfur, giving it tetrahedral molecular geometry and making it nonpolar. SO₃²⁻ (sulfite) has three oxygen atoms and one lone pair on sulfur, giving it trigonal pyramidal geometry and making it polar. Sulfate is more oxidized than sulfite.
What other ions have trigonal pyramidal geometry?
Other trigonal pyramidal species include: NH₃ (ammonia), PO₃³⁻ (phosphite), ClO₃⁻ (chlorite), and H₃O⁺ (hydronium). All have three bonded atoms and one lone pair on the central atom, creating the characteristic pyramid shape.
Can Finish My Math Class help with all chemistry topics?
Yes. We help with all General Chemistry topics: Lewis structures, VSEPR theory, molecular geometry, bonding, stoichiometry, equilibrium, thermodynamics, electrochemistry, and more. We work in ALEKS, MasteringChemistry, Cengage MindTap, and all major platforms. See our Chemistry Homework Help page for details.
Bottom Line: Mastering SO₃²⁻ Molecular Geometry
The molecular geometry of SO₃²⁻ is trigonal pyramidal, shaped by three bonding pairs and one lone pair on the central sulfur atom. Understanding this requires mastering the distinction between electron geometry (tetrahedral—considers all electron domains) and molecular geometry (trigonal pyramidal—considers only bonded atoms).
Key takeaways for SO₃²⁻:
- ✅ 26 total valence electrons (don’t forget the -2 charge)
- ✅ Trigonal pyramidal molecular geometry (not trigonal planar)
- ✅ Tetrahedral electron geometry
- ✅ sp³ hybridization (4 electron domains)
- ✅ Polar molecule (asymmetric shape)
- ✅ ~107.5° bond angles (compressed by lone pair)
- ✅ Three resonance structures with delocalized charge
Whether you’re stuck on this concept or any other general chemistry topic, our chemistry homework help service handles ALEKS, MasteringChemistry, and MindTap assignments with guaranteed high grades.
Need Help with Chemistry Assignments?
We complete ALEKS, MasteringChemistry & MindTap with guaranteed results
High grades guaranteed or your money back
Related Resources: Chemistry Homework Help | A/B Grade Guarantee | All Services | Get a Quote