Shapes of Molecules and Ions | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial provides a comprehensive overview of the shapes of molecules and ions, essential for A-level Chemistry students. Understanding molecular shapes is crucial for predicting reactivity and properties of substances, making this knowledge relevant in both academic and practical chemistry applications.

Step 1: Understanding Electron Pair Repulsion

Electron pairs around a central atom determine the shape of a molecule or ion through repulsion. Here's what to know:

• Bonding Pairs: These are pairs of electrons that participate in bonding. They repel each other equally.
• Lone Pairs: These are non-bonding electron pairs. They exert greater repulsion than bonding pairs due to their higher electron density, reducing bond angles by about 2.5° for each lone pair present.

Practical Tips

• Remember that the more lone pairs present, the more they will affect the overall geometry and bond angles of the molecule.

Step 2: 3D Representation of Molecular Shapes

Molecules have distinct 3D shapes that can be challenging to illustrate. Use the following conventions when drawing:

• Normal Line: Represents a bond in the plane of the paper.
• Dotted Wedge: Indicates a bond going into the paper (away from you).
• Bold Wedge: Represents a bond coming out of the paper (towards you).

Common Pitfalls

• Ensure to correctly represent the 3D arrangement, as misinterpretation can lead to confusion regarding molecular geometry.

Step 3: The Octet Rule and Electron Configuration

The Octet Rule states that atoms tend to bond in a way that results in eight electrons in their outer shell, achieving a noble gas configuration. Consider the following scenarios:

• Shrinking the Octet: Elements like Beryllium and Boron may not have enough electrons to form an octet, leading to compounds such as Boron Trifluoride (BF3).
• Expanding the Octet: Elements in Groups 15-17 (Period 3 and below) can have more than eight electrons. For example, Sulfur in SF6 can form six bonds.

Application

• Recognizing when an element can expand its octet is crucial for predicting its bonding behavior in compounds.

Step 4: Identifying Molecular Shapes

The following shapes correspond to specific bonding pairs and lone pairs:

1. Linear:

   • 2 bonding pairs
   • Examples: H2, CO2
   • Bond angle: 180°

2. Trigonal Planar:

   • 3 bonding pairs
   • Example: BF3
   • Bond angle: 120°

3. Tetrahedral:

   • 4 bonding pairs
   • Example: CH4
   • Bond angle: 109.5°

4. Pyramidal:

   • 3 bonding pairs, 1 lone pair
   • Example: NH3
   • Bond angle: 107°

5. Non-Linear:

   • 2 bonding pairs, 2 lone pairs
   • Example: H2O
   • Bond angle: 104.5°

6. Trigonal Bipyramid:

   • 5 bonding pairs
   • Example: PCl5
   • Bond angles: 90° and 120°

7. Octahedral:

   • 6 bonding pairs
   • Example: SF6
   • Bond angle: 90°

Practical Tips

• Memorize these shapes and their corresponding bond angles, as they are frequently tested in exams.

Conclusion

Understanding molecular shapes and the factors influencing them is vital for A-level Chemistry. Key takeaways include recognizing the importance of electron pair repulsion, mastering 3D representations, and applying the octet rule. To further enhance your knowledge, practice drawing molecular shapes and predicting the geometry of various compounds.