# Shapes of molecules and ions

> IB Chemistry SL · Structure 3: Chemical Bonding
> Source: https://www.owlsprep.com/study/ib-chemistry-sl-u3-shapes-of-molecules-and-ions/

This subtopic explains how to use VSEPR theory to predict the 3D shape, bond angles, and structure of neutral molecules and polyatomic ions from their Lewis structures, including the effect of lone pair repulsion.

**Prerequisites:** [Drawing Lewis structures of molecules and ions](https://www.owlsprep.com/study/ib-chemistry-sl-u3-lewis-structures/); [Covalent bonding basics](https://www.owlsprep.com/study/ib-chemistry-sl-u3-covalent-bonding/)

## Learning objectives

- Apply VSEPR theory to predict shapes of molecules and polyatomic ions
- Calculate steric number from Lewis structures
- Predict and explain bond angle deviations caused by lone pair repulsion
- Distinguish between electron domain geometry and molecular geometry

## VSEPR Theory Fundamentals

**Valence Shell Electron Pair Repulsion (VSEPR) Theory** — A model that predicts the 3D shape of molecules and ions based on the principle that valence electron pairs around a central atom arrange themselves to minimize electrostatic repulsion.

*Example:* VSEPR correctly predicts that methane is tetrahedral, not square planar.

Repulsion between electron domains follows a fixed hierarchy that explains deviations from ideal bond angles: **lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair**. Lone pairs are held closer to the central nucleus, so they occupy more space and exert stronger repulsion.

**Steric Number** — The total number of electron domains around the central atom, calculated as the sum of bonding domains and lone pairs. Any single, double, or triple bond counts as exactly one bonding domain.

*Example:* Ethene's central carbon has 3 bonding domains (one double bond, two single bonds) so steric number = 3.

**Worked example:** Calculate the steric number of the central sulfur atom in sulfur dioxide (SO₂).

1. 1. Draw the Lewis structure of SO₂. The central S is bonded to two O atoms, with one double bond, one single bond, and one lone pair on S.
2. 2. Count electron domains: both bonds count as one domain each, plus one lone pair.
3. 3. Total steric number = 2 bonding domains + 1 lone pair = 3.

> **Exam tip:** Never count each bond in a double or triple bond as a separate domain. This is the most common mistake in Paper 1 multiple choice.

*Calculator:* forbidden

## Common Shapes for Steric Numbers 2–4

For IB SL Chemistry, you are required to know all common shapes for central atoms with steric numbers 2, 3, and 4. Electron domain geometry describes the arrangement of all electron domains (including lone pairs), while molecular geometry describes only the arrangement of atoms.

| Steric Number | Number of Lone Pairs | Electron Geometry | Molecular Shape | Ideal Bond Angle |
| --- | --- | --- | --- | --- |
| 2 | 0 | Linear | Linear | 180° |
| 3 | 0 | Trigonal Planar | Trigonal Planar | 120° |
| 3 | 1 | Trigonal Planar | Bent (V-shaped) | ~117° |
| 4 | 0 | Tetrahedral | Tetrahedral | 109.5° |
| 4 | 1 | Tetrahedral | Trigonal Pyramidal | ~107° |
| 4 | 2 | Tetrahedral | Bent (V-shaped) | ~104.5° |

**Worked example:** Predict the molecular shape and ideal bond angle of the nitrate ion (NO₃⁻).

1. 1. Draw the Lewis structure: central N atom, 3 bonding domains to O atoms, no lone pairs on N.
2. 2. Steric number = 3 bonding domains + 0 lone pairs = 3.
3. 3. For steric number 3 and 0 lone pairs, molecular shape is trigonal planar, ideal bond angle is 120°.

> **Exam tip:** You only need to give bond angles to the nearest degree, or the ideal value unless explicitly asked to explain a deviation.

*Calculator:* forbidden

## VSEPR for Polyatomic Ions

VSEPR rules work exactly the same for polyatomic ions as for neutral molecules. The only difference is that you must account for the ionic charge when counting total valence electrons to draw the correct Lewis structure.

> **info**
>
> Add one electron for every unit of negative charge on an anion, and subtract one electron for every unit of positive charge on a cation, when counting total valence electrons.

**Worked example:** Predict the molecular shape of the hydronium ion (H₃O⁺).

1. 1. Count total valence electrons: O (6) + 3 H (3×1) - 1 (for +1 charge) = 8 total valence electrons.
2. 2. Draw Lewis structure: central O atom, 3 O-H bonds, 1 lone pair on O.
3. 3. Steric number = 3 bonding domains + 1 lone pair = 4.
4. 4. Molecular shape for steric 4, 1 lone pair is trigonal pyramidal.

*Calculator:* forbidden

## Step-by-Step Prediction Method

1. Count total valence electrons, adjusting for ionic charge.
2. Draw the correct Lewis structure, identifying the central atom.
3. Count the number of bonding domains and lone pairs on the central atom.
4. Calculate steric number = bonding domains + lone pairs.
5. Match steric number and lone pair count to get molecular shape and bond angle.

**Worked example:** Explain why the H-N-H bond angle in ammonia (NH₃) is 107°, not 109.5°.

1. 1. The central N atom in NH₃ has steric number 4: 3 bonding domains and 1 lone pair.
2. 2. The ideal tetrahedral bond angle for steric number 4 is 109.5°.
3. 3. The lone pair on N exerts stronger repulsion on the N-H bonding pairs than bonding pairs exert on each other.
4. 4. This stronger repulsion pushes the bonding pairs closer together, reducing the bond angle to ~107°.

> **Exam tip:** Always account for charge first. Missing extra electrons for anions leads to wrong lone pair count and wrong shape.

*Calculator:* forbidden

## Common pitfalls

- **Wrong:** Counting each bond in a double/triple bond as a separate electron domain
  - Why it fails: This gives an incorrect steric number, leading to wrong shape and bond angle
  - Correct: Any covalent bond (single, double, triple) counts as exactly one electron domain
- **Wrong:** Forgetting to adjust valence electron count for ionic charge
  - Why it fails: This leads to incorrect number of lone pairs on the central atom, changing the predicted shape
  - Correct: Add one electron per negative charge, subtract one per positive charge when counting total valence electrons
- **Wrong:** Confusing electron domain geometry with molecular geometry
  - Why it fails: Exam questions almost always ask for molecular geometry (the shape of the atoms, not electron domains)
  - Correct: Name the shape based only on the arrangement of atoms, ignoring lone pairs on the central atom
- **Wrong:** Claiming all steric number 4 molecules have a 109.5° bond angle
  - Why it fails: Lone pairs reduce bond angle, so only molecules with no lone pairs have the exact ideal angle
  - Correct: Use ~107° for trigonal pyramidal and ~104.5° for bent steric 4 molecules
- **Wrong:** Calculating one steric number for molecules with multiple central atoms
  - Why it fails: Each central atom has its own shape based on its own electron count
  - Correct: Predict the shape around each central atom separately for polyatomic molecules with multiple central atoms

## Cheatsheet

| Steric Number | Lone Pairs | Molecular Shape | Bond Angle |
| --- | --- | --- | --- |
| 2 | 0 | Linear | 180° |
| 3 | 0 | Trigonal Planar | 120° |
| 3 | 1 | Bent | ~117° |
| 4 | 0 | Tetrahedral | 109.5° |
| 4 | 1 | Trigonal Pyramidal | ~107° |
| 4 | 2 | Bent | ~104.5° |

## What's next

Understanding molecular shapes is the foundation for predicting molecular polarity, which in turn determines the strength of intermolecular forces and the physical properties (boiling point, solubility, etc.) of compounds. This topic also underpins all of organic chemistry, where the 3D shape of organic molecules determines their reactivity and biological activity. Next, you will extend your knowledge of shapes to learn how shape influences polarity and intermolecular interactions, two of the most heavily tested topics in IB Chemistry SL.

- [Intermolecular Forces](https://www.owlsprep.com/study/ib-chemistry-sl-u3-intermolecular-forces/)
- [Metallic bonding](https://www.owlsprep.com/study/ib-chemistry-sl-u3-metallic-bonding/)

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