VSEPR theory and basic hybridization

IB Chemistry HL· Unit 2: Models of bonding and structure· 20 min read

1. Fundamentals of VSEPR Theory★★☆☆☆⏱ 5 min

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.

📘 Definition

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.

📐 Worked Example

How many electron domains are around the central sulfur atom in the sulfite ion, $S O_{3}^{2 -}$ ?

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

2. Electron Domain vs Molecular Geometry★★★☆☆⏱ 7 min

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 $N H_{3}$ (ammonia).

1
1. Count electron domains around central nitrogen: 3 N-H bonds + 1 lone pair = 4 total electron domains.
2
1. Electron domain geometry for 4 domains is tetrahedral, with an ideal bond angle of 109.5°.
3
1. For molecular geometry, only count bonding domains: 3 bonding domains, 1 lone pair. This corresponds to trigonal pyramidal molecular geometry.
4
1. 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°.

✓ Quick check

Check your understanding of electron domain counting

How many electron domains are around the central carbon atom in carbon dioxide, $C O_{2}$ ?
- 1
- 2
- 3
- 4
Reveal answer
1 —
Correct! Each double bond to oxygen counts as one electron domain, so 2 total domains.

3. Basic Orbital Hybridization★★★☆☆⏱ 5 min

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.

📘 Definition

Hybridization

$s p, s p^{2}, s p^{3}$

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

Example:

One s orbital mixes with three p orbitals to form four equivalent $s p^{3}$ hybrid orbitals.

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

📐 Worked Example

How many unhybridized p orbitals does an $s p$ 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
1. For $s p$ hybridization: 1 s orbital + 1 p orbital are mixed to form 2 equivalent $s p$ hybrid orbitals.
3
1. Subtract orbitals used in hybridization: $4 - 2 = 2$ unhybridized p orbitals remain. These form two π bonds in a triple bond.

4. Relating Steric Number to Hybridization★★★☆☆⏱ 3 min

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	$s p$	Linear
3	$s p^{2}$	Trigonal planar
4	$s p^{3}$	Tetrahedral

📐 Worked Example

Determine the hybridization of the triply bonded carbon atom in propyne, $C H_{3} C \equiv C H$ .

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
1. Steric number 2 corresponds to $s p$ hybridization.

5. Common Pitfalls

Wrong move:

Counting double or triple bonds as multiple electron domains

Why:

VSEPR counts all multiple bonds as a single electron domain, since they occupy the same region of space between two atoms

Correct move:

Count any single, double, or triple bond as one electron domain, regardless of bond order

Wrong move:

Naming electron domain geometry instead of molecular geometry when asked for molecular shape

Why:

Exam questions almost always ask for molecular geometry (arrangement of atoms), which excludes lone pairs from the shape description

Correct move:

Always check if the question asks for electron domain geometry or molecular shape, and only count bonding domains when naming molecular shape

Wrong move:

Assuming all 4-electron-domain molecules have an exact bond angle of 109.5°

Why:

Lone pairs exert greater repulsion than bonding pairs, compressing bond angles below the ideal value

Correct move:

Account for lone pair repulsion: expect bond angles ~2-3° smaller than ideal when lone pairs are present

Wrong move:

Calculating hybridization based on number of bonds instead of number of electron domains

Why:

A central atom with 3 bonds and 1 lone pair has 4 electron domains, so it is $s p^{3}$ hybridized, not $s p^{2}$

Correct move:

Count all electron domains (bonds + lone pairs) to get steric number, then match steric number to hybridization

6. Quick Reference Cheatsheet

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

7. Frequently Asked

What is the difference between electron domain and molecular geometry?

Electron domain geometry counts all electron domains (bonding and lone pairs) around the central atom, while molecular geometry only describes the 3D arrangement of bonded atoms, ignoring lone pairs in the shape name.

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.

2022 · Paper 1
Predict shape of sulfur dioxide
2023 · Paper 2
Identify hybridization in ethene
2021 · Paper 1
VSEPR counting for sulfite ion

Going deeper

Official documentIB Chemistry Data Booklet 2025Use for electronegativity reference when drawing Lewis structures

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 $P C l_{5}$ and $S F_{6}$ , 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.