# VSEPR theory and basic hybridization

> IB Chemistry HL · IB Chemistry HL 2025+
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u2-vsepr-theory-and-basic-hybridization/

This module covers Valence Shell Electron Pair Repulsion (VSEPR) theory to predict molecular shapes, plus basic hybridization of atomic orbitals to explain observed bond angles and geometry for small covalent compounds.

**Prerequisites:** [Drawing Lewis structures of covalent compounds](https://www.owlsprep.com/study/ib-chemistry-hl-u2-lewis-structures/)

## Learning objectives

- Predict molecular geometry and bond angles using VSEPR theory
- Distinguish between electron domain geometry and molecular geometry
- Identify sp, sp² and sp³ hybridization from Lewis structures
- Relate hybridization to predicted molecular shapes from VSEPR

## Fundamentals of VSEPR Theory

VSEPR stands for Valence Shell Electron Pair Repulsion. The core postulate is that valence electron domains (groups of electrons) around a central atom repel each other, and adopt the arrangement that minimizes total repulsion between all electron domains.

**Electron domain** — A region of electron density around a central atom, including any single bond, double bond, triple bond, or lone pair of electrons. Multiple bonds count as a single electron domain.

*Example:* A carbonyl double bond counts as 1 electron domain, not 2.

> **tip**
>
> Repulsion strength follows the order: lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair. This causes predictable deviations from ideal bond angles.

**Worked example:** How many electron domains are around the central sulfur atom in the sulfite ion, $SO_3^{2-}$?

1. 1. Draw the Lewis structure of $SO_3^{2-}$
2. $$\text{Lewis structure: } 
ormalsize{\text{S}}(
ormalsize{=}\text{O})(-\text{O}^-)_2^{2-}\text{ with 1 lone pair on S}$$
3. 2. Count electron domains: 3 bonding domains (one to each oxygen) plus 1 lone pair on sulfur. Multiple bonds count as one domain.
4. 3. Total electron domains = 4

## Electron Domain vs Molecular Geometry

VSEPR distinguishes two types of geometry: electron domain geometry describes the arrangement of all electron domains (bonding and lone pairs), while molecular geometry describes the arrangement of only the bonded atoms. Lone pairs are not included when naming molecular geometry.

**Worked example:** Determine the electron domain geometry, molecular geometry and approximate bond angle for $NH_3$ (ammonia).

1. 1. Count electron domains around central nitrogen: 3 N-H bonds + 1 lone pair = 4 total electron domains.
2. 2. Electron domain geometry for 4 domains is tetrahedral, with an ideal bond angle of 109.5°.
3. 3. For molecular geometry, only count bonding domains: 3 bonding domains, 1 lone pair. This corresponds to trigonal pyramidal molecular geometry.
4. 4. Lone pair-bonding pair repulsion is stronger than bonding pair-bonding repulsion, so the actual H-N-H bond angle is compressed to ~107°, slightly less than 109.5°.

**Check your understanding**

Check your understanding of electron domain counting

1. How many electron domains are around the central carbon atom in carbon dioxide, $CO_2$?

   - 1
   - 2
   - 3
   - 4

   *Answer:* 2

   *Why:* Correct! Each double bond to oxygen counts as one electron domain, so 2 total domains.

## Basic Orbital Hybridization

VSEPR predicts molecular shape but cannot explain why bond angles match experimental observation. For example, carbon in methane has four identical bonds, even though ground state carbon only has 2 unpaired electrons available for bonding. Hybridization resolves this by mixing valence atomic orbitals to form new, identical hybrid orbitals.

**Hybridization** — The mixing of different valence atomic orbitals (s and p) to form equal-energy hybrid orbitals oriented to minimize repulsion, matching VSEPR predicted shapes.

*Notation:* $sp, sp^2, sp^3$

*Example:* One s orbital mixes with three p orbitals to form four equivalent $sp^3$ hybrid orbitals.

- The number of hybrid orbitals formed equals the number of atomic orbitals mixed.
- Hybrid orbitals form sigma ($\sigma$) bonds or hold lone pairs.
- Unhybridized p orbitals form pi ($\pi$) bonds in multiple bonds.

**Worked example:** How many unhybridized p orbitals does an $sp$ hybridized carbon atom have?

1. 1. Carbon has 4 valence atomic orbitals total available for bonding: 1 s orbital + 3 p orbitals = 4 total.
2. 2. For $sp$ hybridization: 1 s orbital + 1 p orbital are mixed to form 2 equivalent $sp$ hybrid orbitals.
3. 3. Subtract orbitals used in hybridization: $4 - 2 = 2$ unhybridized p orbitals remain. These form two π bonds in a triple bond.

## Relating Steric Number to Hybridization

The steric number (equal to the total number of electron domains around the central atom) directly gives the hybridization of the central atom for most small molecules with octet-compliant central atoms.

| Steric Number | Hybridization | Electron Domain Geometry |
| --- | --- | --- |
| 2 | $sp$ | Linear |
| 3 | $sp^2$ | Trigonal planar |
| 4 | $sp^3$ | Tetrahedral |

**Worked example:** Determine the hybridization of the triply bonded carbon atom in propyne, $CH_3C\equiv CH$.

1. 1. Count electron domains around the triply bonded C: it is bonded to two other atoms, with one single bond and one triple bond. Each bond counts as one domain, so 2 total electron domains.
2. 2. Steric number 2 corresponds to $sp$ hybridization.

## Common pitfalls

- **Wrong:** Counting double or triple bonds as multiple electron domains
  - Why it fails: VSEPR counts all multiple bonds as a single electron domain, since they occupy the same region of space between two atoms
  - Correct: Count any single, double, or triple bond as one electron domain, regardless of bond order
- **Wrong:** Naming electron domain geometry instead of molecular geometry when asked for molecular shape
  - Why it fails: Exam questions almost always ask for molecular geometry (arrangement of atoms), which excludes lone pairs from the shape description
  - Correct: Always check if the question asks for electron domain geometry or molecular shape, and only count bonding domains when naming molecular shape
- **Wrong:** Assuming all 4-electron-domain molecules have an exact bond angle of 109.5°
  - Why it fails: Lone pairs exert greater repulsion than bonding pairs, compressing bond angles below the ideal value
  - Correct: Account for lone pair repulsion: expect bond angles ~2-3° smaller than ideal when lone pairs are present
- **Wrong:** Calculating hybridization based on number of bonds instead of number of electron domains
  - Why it fails: A central atom with 3 bonds and 1 lone pair has 4 electron domains, so it is $sp^3$ hybridized, not $sp^2$
  - Correct: Count all electron domains (bonds + lone pairs) to get steric number, then match steric number to hybridization

## Cheatsheet

| Steric Number | Hybridization | Electron Geometry | Molecular Geometry | Ideal Bond Angle |
| --- | --- | --- | --- | --- |
| 2 | $sp$ | Linear | Linear | 180° |
| 3 | $sp^2$ | Trigonal planar | Trigonal planar | 120° |
| 3 (1 LP) | $sp^2$ | Trigonal planar | Bent/V-shaped | ~117° |
| 4 | $sp^3$ | Tetrahedral | Tetrahedral | 109.5° |
| 4 (1 LP) | $sp^3$ | Tetrahedral | Trigonal pyramidal | ~107° |
| 4 (2 LP) | $sp^3$ | Tetrahedral | Bent/V-shaped | ~104.5° |

## What's next

VSEPR and basic hybridization are the foundation for all further study of molecular structure and bonding. Extending these ideas to molecules with expanded octets (more than 4 electron domains around the central atom) will allow you to predict shapes for compounds like $PCl_5$ and $SF_6$, which require d-orbital participation in bonding. Understanding hybridization also leads directly to learning about sigma and pi bonding in organic molecules, which explains the properties of alkenes, including restricted rotation around double bonds. These concepts are also core to understanding intermolecular forces and physical properties of covalent compounds, which depend strongly on molecular polarity and shape.

- [Intermolecular forces and molecular polarity](https://www.owlsprep.com/study/ib-chemistry-hl-u2-intermolecular-forces/)
- [Metallic bonding and alloy structure](https://www.owlsprep.com/study/ib-chemistry-hl-u2-metallic-bonding-and-alloy-structure/)
- [Crystal lattice structures](https://www.owlsprep.com/study/ib-chemistry-hl-u2-crystal-lattice-structures/)

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