Lewis Structure Formula
Visual representation of molecular structure showing valence electrons, bonding pairs, and lone pairs using dot notation
Steps to Draw Lewis Structure
1. Count Total Valence Electrons
2. Arrange Atoms & Draw Bonds
3. Distribute Remaining Electrons
4. Check Formal Charges
Key Concepts
Valence Electrons (V)
Outermost electrons available for bonding (group number for main group elements)
Bonding Pairs (B)
Electron pairs shared between atoms (single, double, triple bonds)
Lone Pairs (N)
Non-bonding electron pairs on an atom
Octet Rule
Atoms tend to gain, lose, or share electrons to achieve 8 valence electrons (2 for H)
Step-by-Step Examples
Example 1: Water (H₂O)
Step 1: Count valence electrons
H: 1 × 2 = 2 electrons, O: 6 electrons → Total = 8 electrons
Step 2: Draw skeleton (O in center, H on sides)
H-O-H (2 bonds = 4 electrons used)
Step 3: Distribute remaining 4 electrons as lone pairs on O
H-O-H with 2 lone pairs on O
Step 4: Check: O has 8 e⁻, each H has 2 e⁻ ✓
Example 2: Carbon Dioxide (CO₂)
Step 1: Count valence electrons
C: 4, O: 6 × 2 = 12 → Total = 16 electrons
Step 2: Draw skeleton with C in center
O-C-O (2 bonds = 4 electrons)
Step 3: Add lone pairs (12 electrons remaining)
Each O gets 6 electrons (3 lone pairs), but C only has 4 e⁻
Step 4: Form double bonds to satisfy octet
O=C=O (each atom has 8 electrons) ✓
Example 3: Ammonium Ion (NH₄⁺)
Step 1: Count valence electrons (subtract charge)
N: 5, H: 1 × 4 = 4, charge: -1 → Total = 8 electrons
Step 2: Draw 4 N-H bonds (8 electrons used)
All electrons used in bonding
Step 3: All atoms satisfy duet/octet ✓
Step 4: Formal charge on N = +1 (matches ion charge) ✓
Common Mistakes to Avoid
Forgetting to Count Charge
Add electrons for negative charges, subtract for positive charges
Violating Octet Rule Unnecessarily
Check if multiple bonds can satisfy octets before accepting expanded octets
Wrong Central Atom
Least electronegative atom (except H) is usually central
Not Checking Formal Charges
Lowest formal charges indicate most stable structure; sum must equal molecular charge
Expanded Octets
Period 3+ elements (P, S, Cl, etc.) can have >8 electrons using d orbitals
Applications
Molecular Geometry
Predict 3D shape using VSEPR theory based on bonding and lone pairs
Polarity
Determine molecular polarity from electronegativity differences and geometry
Reactivity
Lone pairs and formal charges indicate reactive sites in molecules
Resonance
Multiple valid Lewis structures show electron delocalization