Study Guide

Shapes of molecules and ions

IB Chemistry SL· Structure 3.2 Shapes of molecules and ions· 45 min read

1. VSEPR Theory Fundamentals★★☆☆☆⏱ 10 min

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📘 Definition

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.

📘 Definition

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
    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
    1. Count electron domains: both bonds count as one domain each, plus one lone pair.
  3. 3
    1. 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.

2. Common Shapes for Steric Numbers 2–4★★★☆☆⏱ 15 min

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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
    1. Draw the Lewis structure: central N atom, 3 bonding domains to O atoms, no lone pairs on N.
  2. 2
    1. Steric number = 3 bonding domains + 0 lone pairs = 3.
  3. 3
    1. 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.

3. VSEPR for Polyatomic Ions★★★☆☆⏱ 10 min

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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.

📐 Worked Example

Predict the molecular shape of the hydronium ion (H₃O⁺).

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

4. Step-by-Step Prediction Method★★★★☆⏱ 10 min

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

5. Common Pitfalls

Wrong move:

Counting each bond in a double/triple bond as a separate electron domain

Why:

This gives an incorrect steric number, leading to wrong shape and bond angle

Correct move:

Any covalent bond (single, double, triple) counts as exactly one electron domain

Wrong move:

Forgetting to adjust valence electron count for ionic charge

Why:

This leads to incorrect number of lone pairs on the central atom, changing the predicted shape

Correct move:

Add one electron per negative charge, subtract one per positive charge when counting total valence electrons

Wrong move:

Confusing electron domain geometry with molecular geometry

Why:

Exam questions almost always ask for molecular geometry (the shape of the atoms, not electron domains)

Correct move:

Name the shape based only on the arrangement of atoms, ignoring lone pairs on the central atom

Wrong move:

Claiming all steric number 4 molecules have a 109.5° bond angle

Why:

Lone pairs reduce bond angle, so only molecules with no lone pairs have the exact ideal angle

Correct move:

Use ~107° for trigonal pyramidal and ~104.5° for bent steric 4 molecules

Wrong move:

Calculating one steric number for molecules with multiple central atoms

Why:

Each central atom has its own shape based on its own electron count

Correct move:

Predict the shape around each central atom separately for polyatomic molecules with multiple central atoms

6. Quick Reference 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°

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2025 · Paper 1

    Identify shape of sulfate ion

  • 2024 · Paper 2

    Predict shape of NH3 and bond angle

  • 2023 · Paper 1

    Explain bond angle deviation in water

Going deeper

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.