# Shapes of molecules and ions

> CIE A-Level Chemistry · 9701
> Source: https://www.owlsprep.com/study/cie-9701-u3-shapes-of-molecules-and-ions/

This sub-topic introduces Valence Shell Electron Pair Repulsion (VSEPR) theory to predict the 3D shapes of simple covalent molecules and polyatomic ions, including how lone pairs affect bond angles.

**Prerequisites:** [Covalent bonding and Lewis structures](https://www.owlsprep.com/study/cie-9701-u3-covalent-bonding/)

## Learning objectives

- Apply VSEPR theory to predict electron domain and molecular geometries of molecules and ions
- Calculate and explain deviations from ideal bond angles based on electron repulsion
- Distinguish between electron domain geometry and molecular shape
- Predict shapes for species with 2 to 6 electron domains

## VSEPR Theory Fundamentals

**VSEPR Theory** — Valence Shell Electron Pair Repulsion theory states that electron domains (groups of electrons) around a central atom arrange themselves to minimize repulsion between them, producing a predictable 3D shape.

*Notation:* VSEPR

*Example:* A central atom with 2 electron domains will arrange them 180° apart.

Electron domains include both bonding pairs (any single, double or triple bond counts as one domain) and lone pairs of electrons. Repulsion strength follows a clear order: **lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair**. This order explains deviations from ideal bond angles.

**Worked example:** Count the total number of electron domains around the central atom in $NH_3$

1. Identify the central nitrogen atom, which has 5 valence electrons. Each N-H bond uses 1 valence electron from N, so 3 electrons are used in bonding.
2. Remaining valence electrons on N: $5 - 3 = 2$ electrons, which form one lone pair.
3. Total electron domains = 3 bonding domains + 1 lone pair = 4 total domains.

> **Exam tip:** Remember that multiple bonds (double/triple) only count as one electron domain, even though they contain more electrons.

## Common Shapes (2-4 Electron Domains)

For any number of electron domains, there is an ideal electron domain geometry, while molecular shape describes only the positions of the bonded atoms (ignoring lone pairs). 2 to 4 electron domains are the most frequently tested shapes in CIE exams.

**Molecular Geometry** — The 3D arrangement of only bonded atoms around the central atom; lone pairs are not included in the shape name or description.

| Total electron domains | 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) | <120° |
| 4 | 0 | Tetrahedral | Tetrahedral | 109.5° |
| 4 | 1 | Tetrahedral | Trigonal pyramidal | <109.5° |
| 4 | 2 | Tetrahedral | Bent (V-shaped) | <<109.5° |

**Worked example:** Predict the shape and approximate bond angle of the sulfite ion $SO_3^{2-}$

1. Central S atom has 6 valence electrons. Add 2 electrons to account for the 2- negative charge, giving 8 total valence electrons on S.
2. There are 3 S-O bonds, each uses 2 electrons total, leaving 2 unbonded electrons = 1 lone pair.
3. Total electron domains = 3 bonding domains + 1 lone pair = 4.
4. Electron domain geometry is tetrahedral; molecular shape (ignoring the lone pair) is trigonal pyramidal.
5. Lone pair-bonding pair repulsion pushes bonds closer, so the bond angle is ~107°, less than the ideal 109.5°.

## Shapes (5-6 Electron Domains)

Central atoms can have expanded octets (more than 8 valence electrons) to give 5 or 6 total electron domains, with their own characteristic geometries. Lone pairs always occupy positions that minimize repulsion.

| Total electron domains | Lone pairs | Electron geometry | Molecular shape |
| --- | --- | --- | --- |
| 5 | 0 | Trigonal bipyramidal | Trigonal bipyramidal |
| 5 | 1 | Trigonal bipyramidal | See-saw |
| 5 | 2 | Trigonal bipyramidal | T-shaped |
| 5 | 3 | Trigonal bipyramidal | Linear |
| 6 | 0 | Octahedral | Octahedral |
| 6 | 1 | Octahedral | Square pyramidal |
| 6 | 2 | Octahedral | Square planar |

**Worked example:** Predict the shape of xenon tetrafluoride $XeF_4$

1. Central Xe atom has 8 valence electrons. 4 electrons are used in 4 Xe-F bonds.
2. Remaining electrons = $8 - 4 = 4$ electrons = 2 lone pairs.
3. Total electron domains = 4 bonding + 2 lone pairs = 6.
4. Lone pairs occupy opposite axial positions to minimize 90° lone pair-lone pair repulsion.
5. Molecular shape (only atoms) is square planar, with 90° F-Xe-F bond angles.

> **Exam tip:** For 6 electron domains with 2 lone pairs, lone pairs are always opposite each other, never adjacent, to reduce repulsion.

## Shapes of Polyatomic Ions

The same VSEPR rules apply to polyatomic ions, the only adjustment is adding or subtracting electrons to account for the ion's overall charge. Negative charges add electrons to the central atom, while positive charges remove electrons, which changes the number of lone pairs.

**Worked example:** Predict the shape of the ammonium ion $NH_4^+$

1. Central N atom has 5 valence electrons. Subtract 1 electron to account for the +1 charge, giving 4 total valence electrons.
2. All 4 electrons form N-H bonding pairs, so there are 0 lone pairs.
3. Total electron domains = 4 bonding domains, 0 lone pairs = 4 total domains.
4. Molecular shape is tetrahedral, with an ideal bond angle of 109.5°.

## Common pitfalls

- **Wrong:** Counting double or triple bonds as multiple electron domains
  - Why it fails: VSEPR counts any bond between two atoms as a single domain, regardless of bond order
  - Correct: Count every distinct bonded atom to the central atom as one bonding domain
- **Wrong:** Forgetting to adjust the electron count for the charge of an ion
  - Why it fails: Charge changes the number of valence electrons on the central atom, which changes the number of lone pairs
  - Correct: Add 1 electron per negative charge, subtract 1 electron per positive charge when counting valence electrons
- **Wrong:** Calling electron domain geometry the same as molecular shape when lone pairs are present
  - Why it fails: Examiners specifically ask for molecular shape (the shape of atoms only) in most questions
  - Correct: Always state molecular shape unless explicitly asked for electron domain geometry
- **Wrong:** Stating the ideal 109.5° bond angle for water
  - Why it fails: Two lone pairs on oxygen cause greater repulsion than bonding pairs, pushing bonds closer together
  - Correct: State bond angle as ~104.5°, or less than 109.5° if an exact value is not required
- **Wrong:** Placing two lone pairs adjacent in $XeF_4$
  - Why it fails: Adjacent lone pairs have high repulsion at 90°, which is less stable than opposite placement at 180°
  - Correct: Place two lone pairs opposite each other for 6 electron domains with 2 lone pairs

## Cheatsheet

| Total electron domains | 0 lone pairs | 1 lone pair | 2 lone pairs | 3 lone pairs |
| --- | --- | --- | --- | --- |
| 2 | Linear (180°) | - | - | - |
| 3 | Trigonal planar (120°) | Bent (<120°) | - | - |
| 4 | Tetrahedral (109.5°) | Trigonal pyramidal (<109.5°) | Bent (<<109.5°) | - |
| 5 | Trigonal bipyramidal | See-saw | T-shaped | Linear |
| 6 | Octahedral | Square pyramidal | Square planar | - |

## What's next

Mastering VSEPR prediction of molecular and ionic shapes is a high-yield foundational skill for CIE A-Level Chemistry, required for understanding molecular polarity, intermolecular forces, and stereochemistry in organic reactions. Exam questions regularly test your ability to draw shapes, state bond angles, and explain deviations from ideal angles, so consistent practice of this topic is key. Next, you will connect this knowledge to molecular polarity, which depends directly on 3D shape and bond dipole symmetry, and use these shape concepts in later topics like intermolecular forces and organic stereochemistry.

- [Bonding and physical properties](https://www.owlsprep.com/study/cie-9701-u3-bonding-and-physical-properties/)
- [States of matter](https://www.owlsprep.com/study/cie-9701-u4-overview/)
- [Gases and the ideal gas equation](https://www.owlsprep.com/study/cie-9701-u4-gases-and-ideal-gas-equation/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-9701-u3-shapes-of-molecules-and-ions/