Shapes of Molecules

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. State and explain the shapes of, and bond angles in, molecules by using VSEPR theory, including as simple examples: BF₃ (trigonal planar, 120°); CO₂ (linear, 180°); CH₄ (tetrahedral, 109.5°); NH₃ (pyramidal, 107°); H₂O (non-linear, 104.5°); SF₆ (octahedral, 90°); PF₅ (trigonal bipyramidal, 120° and 90°)

  2. Predict the shapes of, and bond angles in, molecules and ions analogous to those specified in 3.5.1

1. Introduction to VSEPR Theory

https://cambridgeassistant.com

VSEPR stands for Valence Shell Electron Pair Repulsion theory. This is a model that helps us predict the 3D shape of molecules.

The Basic Idea

The electrons around the central atom of a molecule exist in pairs. These pairs of electrons behave like negatively charged clouds that repel each other (push each other away) because like charges repel.

To minimize this repulsion (reduce the pushing force), the electron pairs arrange themselves as far apart as possible in three-dimensional space around the central atom.

This arrangement determines the shape of the molecule and the bond angles (the angles between the bonds).

Three Key Rules of VSEPR Theory

  1. All electron pairs spread out as far as possible from each other to minimize repulsion.

  2. Lone pairs repel more strongly than bonding pairs.

    • A bonding pair is a pair of electrons shared between two atoms (forming a bond).
    • A lone pair is a pair of electrons that belongs to only one atom and is not shared.
    • Lone pairs have a more concentrated electron cloud and are closer to the nucleus, so they push other electron pairs away more strongly.

  3. Multiple bonds (double or triple bonds) behave like a single bond when determining shape.

    • For example, a C=O double bond is treated as one region of electron density, just like a single C-O bond.

2. Understanding Repulsion Strength

https://cambridgeassistant.com

The strength of repulsion between electron pairs follows this order:

Lone pair–Lone pair > Lone pair–Bond pair > Bond pair–Bond pair

This means:

  • Lone pairs push each other the hardest (greatest repulsion)
  • Lone pairs push bonding pairs with medium strength (intermediate repulsion)
  • Bonding pairs push each other the least (weakest repulsion)

Why Does This Matter?

When a molecule has lone pairs, they push the bonding pairs closer together, reducing the bond angles from what they would be if all electron pairs were bonding pairs.

Example: Water (H₂O)

  • Water has 2 bonding pairs and 2 lone pairs around the oxygen atom.
  • The two lone pairs push each other very strongly.
  • This pushes the two bonding pairs (the O-H bonds) closer together.
  • Result: The H-O-H bond angle is 104.5° instead of the ideal tetrahedral angle of 109.5°.

Sign in to view full notes