Lewis Structures Overview

Overview

This lecture introduces the basics of Lewis structures, shows step-by-step methods for drawing them for different molecules, and explains key rules and exceptions.

What Are Lewis Structures?

• Lewis structures visually display the 2D connectivity of a molecule using lines for covalent bonds and dots for lone electrons.
• Element symbols represent atoms; lines show shared electron pairs (bonds); dots show lone electron pairs.
• Understanding 2D connectivity helps predict molecular shape and function.

The Octet Rule and Bonding Preferences

• Atoms aim for filled outer s and p subshells (usually 8 electrons), called the octet rule.
• Group 4A elements form 4 bonds; Group 5A elements form 3; Group 6A form 2; Group 7A form 1; Group 8A (noble gases) are inert.
• Knowing an element’s typical number of bonds helps in drawing correct structures.

Steps to Drawing Lewis Structures

1. Count total valence electrons ("building fund").
2. Draw the skeletal structure: choose the central atom (usually the least electronegative or carbon).
3. Complete the octet for non-central atoms by adding lone pairs.
4. Place any remaining electrons on the central atom and ensure all atoms have a full octet or their duet (for H).
5. If the central atom lacks a full octet, convert lone pairs from outer atoms into multiple (double/triple) bonds.

Examples and Special Cases

• Methane (CH₄), water (H₂O), carbon tetrachloride (CCl₄), and phosphorus tribromide (PBr₃) demonstrate basic steps.
• For molecules like CO₂ and SO₃, multiple bonds and resonance structures may be needed.
• Chains of carbons (like propane and butane) are built by connecting carbons in series, then completing bonds with hydrogens.
• For polyatomic ions, add electrons for negative charges and subtract for positive; draw brackets around the structure with the charge indicated.

Resonance and Exceptions

• Resonance structures represent delocalized electrons with double-headed arrows.
• Only elements in period 3 or higher (like sulfur) can exceed the octet rule by using empty d orbitals.
• Elements in period 2 (like C, N, O, F) can never have more than 8 valence electrons.

Key Terms & Definitions

• Lewis structure — a diagram showing valence electrons and bonds in a molecule.
• Octet rule — atoms (except H, He, etc.) seek eight electrons in their valence shell.
• Lone pair — a pair of valence electrons not involved in bonding.
• Resonance structure — alternative ways to draw a molecule, showing delocalized electrons.
• Skeletal structure — the framework of a molecule with atoms and single bonds.
• Polyatomic ion — ion composed of several atoms bonded together, carrying a charge.

Action Items / Next Steps

• Complete assigned homework and draw Lewis structures for given molecules and ions.
• Review octet rule exceptions and resonance structures.
• Practice identifying central atoms and determining correct electron counts.