Lewis Structures & Shapes - Session 3 audio 4/4

Overview

This lecture covers Lewis structures and molecular geometry, focusing on how molecular shapes and electron arrangements determine properties like polarity, solubility, and reactivity.

Lewis Structures and Basic Concepts

• Lewis structures represent molecules by showing atoms, bonds, and non-bonding (lone) electron pairs.
• The shape of a molecule is influenced by the number of terminal atoms and non-bonding electron pairs on the central atom.
• Lone pairs are non-bonding electrons on the central atom that affect molecular geometry.

Common Molecular Shapes

• Linear: All atoms are in a straight line (bond angle 180°); examples: HF, CO, CO₂, BeH₂.
• Trigonal Planar: Flat, three bonds to central atom, bond angle 120°; example: BF₃ (rare, boron is an exception to the octet rule).
• Tetrahedral: Four terminal atoms around a central atom, 3D structure, bond angle 109.5°; example: CH₄.
• Trigonal Pyramidal: Three terminal atoms and one lone pair on central atom, 3D structure; example: NH₃.
• Bent (Angular): Two terminal atoms and one or two lone pairs on central atom, bond angle ~104.5°; example: H₂O.

Effects of Lone Pairs on Molecular Shape

• Non-bonding electron pairs repel more strongly than bonded atoms, causing shapes like bent or trigonal pyramidal.
• Lone pairs distort bond angles, making them smaller than ideal geometries (e.g., water's 104.5°).

Polarity and Molecular Properties

• Polar molecules have unequal sharing of electrons, creating partial positive and negative charges (e.g., H₂O, NH₃, HF).
• Non-polar molecules have equal sharing and are symmetrical (e.g., CH₄).
• Polarity affects solubility: "like dissolves like"—polar compounds dissolve in polar solvents, non-polar in non-polar solvents.
• Polar covalent compounds have higher boiling/melting points than non-polar ones.

Exceptions & Advanced Shapes

• Some elements (e.g., S, P) can expand their octet, holding more than 8 valence electrons.
• Complex shapes like trigonal bipyramidal and octahedral exist but are not the focus here.

Key Terms & Definitions

• Lewis Structure — Diagram showing bonds and lone pairs in a molecule.
• Lone Pair (Non-bonding Pair) — Pair of valence electrons not shared or bonded.
• Terminal Atom — Atom bonded to the central atom in a molecule.
• Bond Angle — Angle between two bonds originating from the same atom.
• Polarity — Distribution of electrical charge over the atoms in a molecule.
• Tetrahedral — 3D molecular shape with four bonds at 109.5° angles.
• Trigonal Planar — Flat shape with three bonds at 120° angles.
• Trigonal Pyramidal — 3D shape with three bonds and a lone pair.
• Bent/Angular — Molecule with two bonds and lone pairs causing a bent shape.

Action Items / Next Steps

• Practice drawing Lewis structures for small molecules and ions.
• Memorize shapes: linear, trigonal planar, tetrahedral, trigonal pyramidal, bent/angular.
• Review the concept of polarity and predict if a molecule is polar or non-polar.
• Read textbook sections on molecular geometry and polarity.
• Use provided diagrams/models for visualization.