VSEPR Theory

Q: What is the primary assumption of VSEPR theory?

The primary assumption of VSEPR theory is that electron pairs in the valence shell of a central atom, whether they are involved in bonding (bond pairs) or not (lone pairs), will repel each other. To minimize these repulsive forces and achieve the most stable arrangement, these electron pairs will orient themselves as far apart as possible in three-dimensional space. This fundamental principle dictates the electron domain geometry, which then informs the molecular geometry.

Q: How do lone pairs affect molecular geometry and bond angles?

Lone pairs significantly influence molecular geometry and bond angles because they occupy more space and exert stronger repulsive forces than bond pairs. Unlike bond pairs, which are shared between two nuclei, lone pairs are attracted to only one nucleus, allowing them to spread out more. This increased repulsion from lone pairs pushes bonding pairs closer together, causing a distortion from the ideal electron domain geometry and leading to a reduction in bond angles. For example, water ($H_2O$) has two lone pairs on oxygen, which compress the $H-O-H$ bond angle from the ideal tetrahedral $109.5^\circ$ to approximately $104.5^\circ$.

Q: What is the difference between electron domain geometry and molecular geometry?

Electron domain geometry describes the arrangement of *all* electron groups (both bonding pairs and lone pairs) around the central atom. It's determined solely by the steric number. Molecular geometry, on the other hand, describes the arrangement of *only the atoms* in a molecule. While electron domain geometry considers lone pairs as part of the spatial arrangement, molecular geometry focuses on the visible shape formed by the nuclei. If there are no lone pairs, electron domain geometry and molecular geometry are identical. If lone pairs are present, they influence the molecular geometry but are not part of its description.

Q: Why are multiple bonds treated as a single electron domain in VSEPR theory?

In VSEPR theory, double and triple bonds are treated as a single electron domain (or electron group) because all the electrons in a multiple bond are localized between the same two atoms. Even though a double bond has four electrons and a triple bond has six, they effectively occupy the same region of space between the two bonded atoms. Therefore, they exert repulsion as a single unit, influencing the overall geometry in the same way a single bond would, albeit with potentially slightly stronger repulsion due to higher electron density.

Q: Can VSEPR theory predict the geometry of ionic compounds?

VSEPR theory is primarily designed for predicting the geometry of covalent molecules and polyatomic ions, where there is a distinct central atom and localized electron pairs. It is generally not applied to extended ionic compounds (like NaCl lattice) because these compounds consist of a crystal lattice of ions rather than discrete molecules with a central atom. However, for polyatomic ions (e.g., $SO_4^{2-}$, $NH_4^+$), VSEPR theory is perfectly applicable as they behave like molecules with covalent bonds and a central atom.

Q: How do you determine the steric number for an ion?

To determine the steric number for an ion, you use a slightly modified formula. For a polyatomic anion, you add the magnitude of the negative charge to the total valence electrons of the central atom. For a polyatomic cation, you subtract the magnitude of the positive charge. The general formula for steric number (SN) is: $SN = \frac{1}{2} (\text{Valence electrons of central atom} + \text{Number of monovalent atoms} \pm \text{Charge})$. For example, for $ClO_3^-$: Central atom Cl has 7 valence electrons. O is divalent, so it doesn't count as monovalent. The charge is -1, so we add 1. $SN = \frac{1}{2} (7 + 0 + 1) = 4$. For $NH_4^+$: Central atom N has 5 valence electrons. H is monovalent (4 H atoms). The charge is +1, so we subtract 1. $SN = \frac{1}{2} (5 + 4 - 1) = 4$.

Updated 22 Mar 2026

Personal Mentor

Stuck? Get it explained, or see what to know first.

Explains step by step, tests you, compares — from this topic’s own material.

Preparation Manager

Next up: Kossel-Lewis Approach to Chemical Bonding Preparation Manager

Revision & Notes

Textbook Steps Glance

Revision Manager

Mindscaping

Know your depth on VSEPR Theory: 5 questions, 3 minutes, a plan.

Check my depth Sign in to keep your score.

Sub-topics

1 sub-topics

1Molecular GeometryHigh yield

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is based on the premise that the valence shell electron pairs (both bonding and non-bonding) around a central atom will arrange themselves as far apart as possible to minimize electrostatic repuls…

Quick Summary

VSEPR theory is a simple yet powerful model to predict molecular shapes. It's based on the idea that electron pairs (both bonding and non-bonding, or lone pairs) around a central atom repel each other and arrange themselves to minimize this repulsion.

The first step is to draw the Lewis structure to identify the central atom and count its valence electron pairs. Each single, double, or triple bond counts as one 'electron domain', and each lone pair also counts as one 'electron domain'.

The total number of electron domains determines the 'electron domain geometry' (e.g., 2 domains = linear, 3 = trigonal planar, 4 = tetrahedral). The 'molecular geometry' is then determined by the arrangement of atoms only.

Lone pairs exert stronger repulsion than bonding pairs, leading to distortions in bond angles and affecting the final molecular shape. For instance, $CH_4$ (4 bond pairs, 0 lone pairs) is tetrahedral, $NH_3$ (3 bond pairs, 1 lone pair) is trigonal pyramidal, and $H_2O$ (2 bond pairs, 2 lone pairs) is bent, all stemming from a tetrahedral electron domain geometry but differing in molecular geometry due to lone pair influence.

Full explanation

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a powerful yet simple model that allows chemists to predict the three-dimensional arrangement of atoms in a molecule, commonly referred to as its molecular geometry. This theory is built upon the fundamental principle that electron pairs in the valence shell of a central atom will repel each each other and thus orient themselves in space to minimize these repulsive forces, achieving the most stable configuration.

Conceptual Foundation:

At the heart of VSEPR theory is the concept of 'electron domains' or 'electron groups'. An electron domain can be a single bond, a double bond, a triple bond, or a lone pair of electrons. Crucially, a multiple bond (double or triple) is treated as a single electron domain because all electrons in that bond are localized between the same two atoms.

The central atom is the atom to which all other atoms are directly bonded. The total number of electron domains around the central atom determines the 'electron domain geometry'.

Key Principles/Postulates of VSEPR Theory:

Electron pairs repel: — All electron pairs in the valence shell of the central atom, whether bonding or non-bonding (lone pairs), repel each other.

Minimization of repulsion: — These electron pairs arrange themselves in space such that the repulsion between them is minimized, leading to the most stable geometry.

Electron domain geometry vs. Molecular geometry: — The arrangement of electron domains around the central atom defines the 'electron domain geometry'. However, the 'molecular geometry' (the shape defined by the positions of the atomic nuclei) is determined by the positions of only the bonding electron pairs. Lone pairs influence the molecular geometry by their repulsive forces but are not considered part of the visible shape.

Order of repulsion: — The repulsive forces between electron pairs follow a specific order:

Lone pair - Lone pair (LP-LP) repulsion is strongest. Lone pair - Bond pair (LP-BP) repulsion is intermediate. * Bond pair - Bond pair (BP-BP) repulsion is weakest. This order is critical because lone pairs occupy more space around the central atom than bonding pairs, as they are attracted to only one nucleus and are thus more diffuse.

This greater spatial requirement and stronger repulsion by lone pairs cause distortions in the ideal bond angles predicted by electron domain geometry alone.

Steps to Predict Molecular Geometry using VSEPR Theory:

Draw the Lewis Structure: — This is the foundational step. Correctly drawing the Lewis structure identifies the central atom, the number of bonding pairs, and the number of lone pairs on the central atom.

Count Electron Domains (Steric Number): — Determine the total number of electron domains around the central atom. Remember, each single, double, or triple bond counts as one domain, and each lone pair counts as one domain. This sum is often called the steric number.

Determine Electron Domain Geometry: — Based on the steric number, predict the arrangement of electron domains that minimizes repulsion:

* Steric Number 2: Linear ( $180^\circ$ ) * Steric Number 3: Trigonal Planar ( $120^\circ$ ) * Steric Number 4: Tetrahedral ( $109.5^\circ$ ) * Steric Number 5: Trigonal Bipyramidal (axial $90^\circ$ , equatorial $120^\circ$ ) * Steric Number 6: Octahedral ( $90^\circ$ )

Determine Molecular Geometry: — Now, consider the number of lone pairs. The molecular geometry is determined by the arrangement of the atoms only. Lone pairs occupy positions in the electron domain geometry but are 'invisible' when describing the molecular shape. The presence of lone pairs will often distort the ideal bond angles due to their stronger repulsive forces.

* Steric Number 2 (0 LP): Linear (e.g., $CO_2$ ) * Steric Number 3 (0 LP): Trigonal Planar (e.g., $BF_3$ ) * Steric Number 3 (1 LP): Bent / V-shaped (e.g., $SO_2$ ) * Steric Number 4 (0 LP): Tetrahedral (e.

g., $CH_4$ ) * Steric Number 4 (1 LP): Trigonal Pyramidal (e.g., $NH_3$ ) * Steric Number 4 (2 LP): Bent / V-shaped (e.g., $H_2O$ ) * Steric Number 5 (0 LP): Trigonal Bipyramidal (e.g., $PCl_5$ ) * Steric Number 5 (1 LP): See-Saw (e.

g., $SF_4$ ) * Steric Number 5 (2 LP): T-shaped (e.g., $ClF_3$ ) * Steric Number 5 (3 LP): Linear (e.g., $XeF_2$ ) * Steric Number 6 (0 LP): Octahedral (e.g., $SF_6$ ) * Steric Number 6 (1 LP): Square Pyramidal (e.

g., $BrF_5$ ) * Steric Number 6 (2 LP): Square Planar (e.g., $XeF_4$ ) * Steric Number 7 (0 LP): Pentagonal Bipyramidal (e.g., $IF_7$ ) * Steric Number 7 (1 LP): Pentagonal Pyramidal (e.g., $XeOF_5^-$ ) * Steric Number 7 (2 LP): Pentagonal Planar (e.

Predict Bond Angles: — Account for the effect of lone pairs. Lone pairs reduce bond angles from the ideal values. For example, in

CH_4

(0 LP), the bond angle is

109.5^\circ

. In

NH_3

(1 LP), it's

107^\circ

. In

H_2O

(2 LP), it's

104.5^\circ

. This trend clearly illustrates the increasing LP-BP repulsion.

Real-World Applications:

VSEPR theory is fundamental to understanding molecular properties. The shape of a molecule dictates its polarity, which in turn affects its solubility, boiling point, and melting point. For instance, a linear molecule like $CO_2$ is nonpolar even though its individual C=O bonds are polar, because the bond dipoles cancel out.

Water ( $H_2O$ ), with its bent shape, is highly polar because its O-H bond dipoles do not cancel, leading to its unique properties as a solvent and its high boiling point compared to similar-sized nonpolar molecules.

Molecular shape is also critical in biological systems, where the precise fit between molecules (like enzymes and substrates, or drugs and receptors) is determined by their three-dimensional structures.

Common Misconceptions:

Confusing electron domain geometry with molecular geometry: — Students often forget that lone pairs influence the shape but are not part of the 'visible' molecular geometry. Always distinguish between the arrangement of all electron domains and the arrangement of atoms only.

Incorrectly counting electron domains: — A common error is counting double or triple bonds as two or three separate domains instead of a single domain.

Ignoring lone pairs: — Sometimes, students forget to include lone pairs on the central atom when calculating the steric number, leading to an incorrect electron domain geometry and subsequent molecular geometry.

Applying VSEPR to non-central atoms: — VSEPR theory is primarily used to predict the geometry around a central atom. While it can be extended to predict local geometries around multiple central atoms in larger molecules, it's not for predicting the geometry of terminal atoms.

Overlooking the repulsion order: — Not understanding that LP-LP > LP-BP > BP-BP repulsion is crucial for explaining bond angle distortions.

NEET-Specific Angle:

For NEET, VSEPR theory is a high-yield topic. Questions frequently involve:

Predicting the molecular geometry/shape — of a given molecule or ion (e.g.,

XeF_4

I_3^-

SF_4

ClO_3^-

Comparing bond angles — in related molecules (e.g.,

CH_4

NH_3

H_2O

Identifying molecules with similar shapes — or electron domain geometries.

Relating molecular geometry to polarity — (e.g., which of the following is polar/nonpolar?).

Understanding exceptions or special cases, such as molecules where steric factors or d-orbital participation might slightly modify predictions (though these are less common in basic NEET questions).

A quick and accurate method for determining the steric number is essential. For a neutral molecule, it's $\frac{1}{2} (\text{Valence electrons of central atom} + \text{Number of monovalent atoms} - \text{Charge if anion} + \text{Charge if cation})$ .

For example, in $NH_3$ : $\frac{1}{2} (5 + 3 - 0 + 0) = 4$ . Steric number 4 with 3 bond pairs (N-H) means 1 lone pair, leading to trigonal pyramidal geometry. Mastering this shortcut and the common geometries associated with different steric numbers and lone pair counts will significantly improve speed and accuracy in the exam.

Derivations (Not applicable for VSEPR):

VSEPR theory is a predictive model based on empirical observations and electrostatic principles, not derived from quantum mechanics like Valence Bond Theory or Molecular Orbital Theory. Its strength lies in its simplicity and effectiveness in predicting molecular shapes without complex calculations.

Summary of Geometries based on Steric Number (SN) and Lone Pairs (LP):

SN	LP	Bond Pairs	Electron Geometry	Molecular Geometry	Example	Bond Angle (approx.)
2	0	2	Linear	Linear	$CO_2$ , $BeCl_2$	$180^\circ$
3	0	3	Trigonal Planar	Trigonal Planar	$BF_3$ , $SO_3$	$120^\circ$
3	1	2	Trigonal Planar	Bent / V-shaped	$SO_2$ , $O_3$	$<120^\circ$
4	0	4	Tetrahedral	Tetrahedral	$CH_4$ , $SiCl_4$	$109.5^\circ$
4	1	3	Tetrahedral	Trigonal Pyramidal	$NH_3$ , $PCl_3$	$107^\circ$
4	2	2	Tetrahedral	Bent / V-shaped	$H_2O$ , $H_2S$	$104.5^\circ$
5	0	5	Trigonal Bipyramidal	Trigonal Bipyramidal	$PCl_5$ , $AsF_5$	$90^circ, 120^\circ$
5	1	4	Trigonal Bipyramidal	See-Saw	$SF_4$ , $TeCl_4$	$<90^circ, <120^\circ$
5	2	3	Trigonal Bipyramidal	T-shaped	$ClF_3$ , $BrF_3$	$<90^\circ$
5	3	2	Trigonal Bipyramidal	Linear	$XeF_2$ , $I_3^-$	$180^\circ$
6	0	6	Octahedral	Octahedral	$SF_6$ , $TeF_6$	$90^\circ$
6	1	5	Octahedral	Square Pyramidal	$BrF_5$ , $IF_5$	$<90^\circ$
6	2	4	Octahedral	Square Planar	$XeF_4$ , $ICl_4^-$	$90^\circ$

This systematic approach makes VSEPR theory an indispensable tool for understanding and predicting molecular structures in chemistry.

Key Concepts

Steric Number and Electron Domain Geometry

The steric number (SN) is the sum of the number of atoms directly bonded to the central atom and the number…

Impact of Lone Pairs on Molecular Geometry

While electron domain geometry is determined by the total number of electron domains, molecular geometry is…

Predicting Bond Angles with VSEPR

VSEPR theory not only predicts the general shape but also helps in estimating bond angles. The ideal bond…

Often confused with

Side-by-side differences the NEET paper likes to test.

VSEPR Theory vs Valence Bond Theory (VBT)

Aspect	VSEPR Theory	Valence Bond Theory (VBT)
Fundamental Principle	VSEPR Theory: Electron pairs repel each other and arrange to minimize repulsion.	Valence Bond Theory: Covalent bonds form by the overlap of atomic orbitals, leading to electron sharing.
Focus	VSEPR Theory: Primarily predicts molecular geometry and bond angles based on electron domain arrangement.	Valence Bond Theory: Explains bond formation, bond strength, and hybridization of orbitals to account for geometry.
Role of Lone Pairs	VSEPR Theory: Lone pairs are treated as electron domains that exert stronger repulsion, distorting molecular geometry and bond angles.	Valence Bond Theory: Lone pairs occupy hybridized orbitals but their direct repulsive effect on bond angles is less explicitly quantified compared to VSEPR.
Predictive Power	VSEPR Theory: Excellent for predicting shapes of simple molecules and polyatomic ions, especially with lone pairs.	Valence Bond Theory: Explains bonding and geometry through hybridization, but sometimes struggles with precise bond angles without VSEPR's repulsion rules.
Complexity	VSEPR Theory: Simpler, qualitative model, easy to apply.	Valence Bond Theory: More complex, involves understanding orbital overlap and hybridization.

VSEPR theory and Valence Bond Theory (VBT) are both crucial for understanding molecular structure but approach it from different angles. VSEPR is a qualitative model focused on minimizing electron pair repulsion to predict molecular geometry, especially effective in explaining bond angle distortions due to lone pairs.

VBT, on the other hand, is a more quantitative theory that explains bond formation through the overlap of atomic orbitals and the concept of hybridization, which accounts for the observed geometries. While VSEPR tells us what the shape is likely to be, VBT explains how that shape arises from orbital interactions.

Both are complementary in providing a complete picture of molecular structure.

Why it is tested: For NEET, both VSEPR and VBT are highly relevant. VSEPR is frequently tested for direct prediction of molecular shapes, bond angles, and polarity. VBT is crucial for understanding hybridization and its correlation with geometry. Questions often require applying principles from both theories, for example, identifying the hybridization and then using VSEPR to refine the molecular geometry, especially when lone pairs are present. Understanding their differences helps in choosing the appropriate model for a given problem.

Questions students ask

6 answered on this topic.

What is the primary assumption of VSEPR theory?

The primary assumption of VSEPR theory is that electron pairs in the valence shell of a central atom, whether they are involved in bonding (bond pairs) or not (lone pairs), will repel each other. To minimize these repulsive forces and achieve the most stable arrangement, these electron pairs will orient themselves as far apart as possible in three-dimensional space. This fundamental principle dictates the electron domain geometry, which then informs the molecular geometry.

How do lone pairs affect molecular geometry and bond angles?

Lone pairs significantly influence molecular geometry and bond angles because they occupy more space and exert stronger repulsive forces than bond pairs. Unlike bond pairs, which are shared between two nuclei, lone pairs are attracted to only one nucleus, allowing them to spread out more.

This increased repulsion from lone pairs pushes bonding pairs closer together, causing a distortion from the ideal electron domain geometry and leading to a reduction in bond angles. For example, water ( $H_2O$ ) has two lone pairs on oxygen, which compress the $H-O-H$ bond angle from the ideal tetrahedral $109.

5^\circ $to approximately$ 104.5^\circ$.

What is the difference between electron domain geometry and molecular geometry?

Electron domain geometry describes the arrangement of all electron groups (both bonding pairs and lone pairs) around the central atom. It's determined solely by the steric number. Molecular geometry, on the other hand, describes the arrangement of only the atoms in a molecule.

While electron domain geometry considers lone pairs as part of the spatial arrangement, molecular geometry focuses on the visible shape formed by the nuclei. If there are no lone pairs, electron domain geometry and molecular geometry are identical.

If lone pairs are present, they influence the molecular geometry but are not part of its description.

Why are multiple bonds treated as a single electron domain in VSEPR theory?

In VSEPR theory, double and triple bonds are treated as a single electron domain (or electron group) because all the electrons in a multiple bond are localized between the same two atoms. Even though a double bond has four electrons and a triple bond has six, they effectively occupy the same region of space between the two bonded atoms.

Therefore, they exert repulsion as a single unit, influencing the overall geometry in the same way a single bond would, albeit with potentially slightly stronger repulsion due to higher electron density.

Can VSEPR theory predict the geometry of ionic compounds?

VSEPR theory is primarily designed for predicting the geometry of covalent molecules and polyatomic ions, where there is a distinct central atom and localized electron pairs. It is generally not applied to extended ionic compounds (like NaCl lattice) because these compounds consist of a crystal lattice of ions rather than discrete molecules with a central atom.

However, for polyatomic ions (e.g., $SO_4^{2-}$ , $NH_4^+$ ), VSEPR theory is perfectly applicable as they behave like molecules with covalent bonds and a central atom.

How do you determine the steric number for an ion?

To determine the steric number for an ion, you use a slightly modified formula. For a polyatomic anion, you add the magnitude of the negative charge to the total valence electrons of the central atom.

For a polyatomic cation, you subtract the magnitude of the positive charge. The general formula for steric number (SN) is: $SN = \frac{1}{2} (\text{Valence electrons of central atom} + \text{Number of monovalent atoms} \pm \text{Charge})$ .

For example, for $ClO_3^-$ : Central atom Cl has 7 valence electrons. O is divalent, so it doesn't count as monovalent. The charge is -1, so we add 1. $SN = \frac{1}{2} (7 + 0 + 1) = 4$ . For $NH_4^+$ : Central atom N has 5 valence electrons.

H is monovalent (4 H atoms). The charge is +1, so we subtract 1. $SN = \frac{1}{2} (5 + 4 - 1) = 4$ .

Revise in 30 seconds

VSEPR Principle: — Electron pairs repel, arrange to minimize repulsion.

Electron Domains: — Single, double, triple bonds, and lone pairs each count as one domain.

Steric Number (SN): — Sum of bonded atoms + lone pairs on central atom.

Repulsion Order: — LP-LP > LP-BP > BP-BP (Lone Pair > Bond Pair).

Key Geometries (SN, LP):

- SN 2, 0 LP: Linear ( $180^\circ$ ) - SN 3, 0 LP: Trigonal Planar ( $120^\circ$ ) - SN 3, 1 LP: Bent ( $<120^\circ$ ) - SN 4, 0 LP: Tetrahedral ( $109.5^\circ$ ) - SN 4, 1 LP: Trigonal Pyramidal ( $107^\circ$ ) - SN 4, 2 LP: Bent ( $104.5^\circ$ ) - SN 5, 0 LP: Trigonal Bipyramidal ( $90^circ, 120^\circ$ ) - SN 5, 1 LP: See-Saw - SN 5, 2 LP: T-shaped - SN 5, 3 LP: Linear - SN 6, 0 LP: Octahedral ( $90^\circ$ ) - SN 6, 1 LP: Square Pyramidal - SN 6, 2 LP: Square Planar

Formula for SN (neutral): —

SN = \frac{1}{2} (\text{Valence e}^- \text{ of central atom} + \text{No. of monovalent atoms})

Formula for SN (ion): —

SN = \frac{1}{2} (\text{Valence e}^- \text{ of central atom} + \text{No. of monovalent atoms} \pm \text{Charge})

To remember the repulsion order: Lone Lone Bond Bond. (LP-LP > LP-BP > BP-BP). Think of 'L' as 'Large' repulsion and 'B' as 'Small' repulsion. So, Large-Large > Large-Small > Small-Small.