A Level Chemistry Revision "Shapes of Molecules".

Introduction

This tutorial covers the shapes of molecules as part of A Level Chemistry revision. Understanding molecular geometry is crucial for predicting the behavior and properties of substances. We will explore electron pair repulsion theory and how it determines the shapes of various molecules without lone pairs on the central atom.

Step 1: Understand Electron Pair Repulsion Theory

• Electron pairs around a central atom repel each other.
• The arrangement of these pairs minimizes repulsion, leading to specific molecular shapes.
• Key concepts include:
  • Bond pairs: Pairs of electrons shared between atoms.
  • Lone pairs: Pairs of electrons that are not involved in bonding.

Step 2: Explore Molecular Shapes

Learn about the common shapes of molecules based on the number of bond pairs:

Linear Shape

• Geometry: 2 bond pairs
• Bond Angle: 180°
• Example: Carbon dioxide (CO₂)

Trigonal Planar Shape

• Geometry: 3 bond pairs
• Bond Angle: 120°
• Example: Boron trifluoride (BF₃)

Tetrahedral Shape

• Geometry: 4 bond pairs
• Bond Angle: 109.5°
• Example: Methane (CH₄)

Trigonal Bipyramidal Shape

• Geometry: 5 bond pairs
• Bond Angles: 90° and 120°
• Example: Phosphorus pentachloride (PCl₅)

Octahedral Shape

• Geometry: 6 bond pairs
• Bond Angle: 90°
• Example: Sulfur hexafluoride (SF₆)

Step 3: Analyze Multiple Bonds

• Multiple bonds (like double bonds) affect molecular geometry.
• Double bonds count as one electron pair when determining shape.
• For example, in carbon dioxide (CO₂), the double bonds influence its linear shape.

Step 4: Practice Drawing Molecular Shapes

• Use Lewis structures to visualize molecular shapes.
• Identify the number of bond pairs and assign the correct shape based on the guidelines above.
• Practice with various molecules to reinforce your understanding.

Conclusion

Understanding the shapes of molecules is essential for A Level Chemistry. Remember the key shapes and their corresponding bond angles: linear (180°), trigonal planar (120°), tetrahedral (109.5°), trigonal bipyramidal (90° and 120°), and octahedral (90°). For further study, review molecules with lone pairs in the next video and continue practicing with Lewis structures to solidify your knowledge.