Valence-Shell Electron-Pair Repulsion (VSEPR) Theory/VESPER Theory

Welcome to my chemistry corner, where molecules arrange themselves like characters forming patterns on a cosmic stage. I’ve crafted this guide to help you understand how atoms decide their shapes, angles, and geometry using the VSEPR model. Whether you’re preparing for MDCAT, ECAT, board exams, or revising your basics, this post will give you clarity, confidence, and exam-ready insight.

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🔷✨ VSEPR or VESPER Theory Explained | Shape of Molecules Made Easy | Bond Angles, Electron Pairs, Geometry Guide (Sidgwick-Powell Theory/ Gillespie-Nyholm Theory)

🌈🔥 Theories of Shape of the Molecules

Simple polyatomic molecules and ions generally acquire linear, trigonal, tetrahedral, pyramidal and angular shapes etc. These shapes can be determined experimentally, however also predicted on theoretical basis. Besides VBT, there are two more significant theories which describe the shape of molecules

1. Valence shell electron pair repulsion theory (VSEPR or VESPER)

2. Hybridization or Hybrid orbital model

🌈🔥Introduction

This theory was proposed for the first time by Sidgwick and Powell in 1940 and was developed by Gillespie and Nyholm in 1957.This theory helps to predict the shape or geometry of simple covalent molecules and ions of non-transition elements i.e. it throws light on the three dimensional shapes of molecules on the basis of electron pairs orientation present on central atom. VSEPR is based upon minimizing the extent of the electron-pair repulsion around the central atom being considered.

This theory is based on electron pairs repulsion, that is why it is named as valence-shell electron pair repulsion theory denoted by VSEPR (the acronym “VSEPR” is pronounced as vesper). It is also known as electron pair repulsion model denoted as EPRM.

Molecular geometry is associated with the specific orientation of bonding in atoms. A careful analysis of electron distributions through the writing of Lewis diagrams will usually result in correct electron pair and molecular geometry determinations.

🌈🔥Statement

The active electron pairs (both shared pairs and lone pairs) surrounding the central atom will be arranged in space as far apart as possible to minimize the electrostatic inter-electronic repulsion giving a definite shape to the molecule. OR The valence electron pairs both bond pair and lone pair are arranged around the central atom to remain at maximum distance apart to keep repulsion at a minimum.

The electron pairs around the central atom, whether shared (bond pair) or unshared (lone pair) are called active sets of electron pairs. Pi electrons are not considered to be active set of electron pair.

🌈🔥Basis of Theory

This theory was based on electron pairs repulsion of the valence shell of central atom which is responsible to give characteristic shape of molecules. The concept tells that, the arrangement of bonds around the central atom depends upon the repulsion’s operating between electron pairs (bonded or non-bonded) around the central atom.

🌈🔥Main Assumptions

1. Active Electrons pair or Active pair or steric number
2. Repulsion between Active pairs of electrons
3. Orientation of active pairs at maximum separation and minimum repulsion
4. Order of repulsions between active pairs
5. Relation between repulsion forces and bond angles
6. Reduction in bond angle due to lone pairs
7. Relation between molecular geometry and active electron pairs

🌿⚛️1. There may be two types of electron pairs surrounding the central atom.

(a) Bond Pairs; The electrons which take part either in single, double or triple bond formation (bond pair) between the central atom and surrounding atom are considered to be one pair of Active Electrons. These are the result of the sharing of unpaired electrons of central atom with unpaired electrons of surrounding atoms. They are also active set of electrons.

(b) Lone Pairs; These are the paired electrons of central atom which do not take part in sharing. They are also called non-bonding pairs. It is also considered as active set of electrons.

The sum of bond pair and lone pair are collectively called active pair or steric number.

In case of molecules with multiple bonds in the form of double and triple bonds, the pi electrons are considered to be inactive set of electrons in VESPER because pi bonds do not alter the basic idealized geometry of a molecules. Hence pi electrons are not included in the count of total active electron pairs.

🌿⚛2. being similarly charged i.e. negative, the bond pairs as well as the lone pairs repel each other.

🌿⚛3. Due to repulsion, the electrons pairs of central atom try to be as far as apart as possible; hence the orient themselves in space in such a manner that force of repulsion between them is minimized

🌿⚛4. The force of repulsion between lone pairs and bond pairs is not the same. (Since lone pairs are spread out more broadly than the bonding pairs, repulsion is greatest between two lone pairs, intermediate between the lone pair and bonding pair and weakest between two bonding pairs). The order of repulsion is:

Lone Pair-Lone Pair repulsion > Lone Pair-Bond Pair repulsion> Bond Pair-Bond Pair repulsion

🌿⚛5. Repulsion forces decrease sharply with increasing bond angle or inter-pair angle (i.e. angle between two bond pairs).They are strong at 90°, much weaker at 120° and very weak at 180°.

🌿⚛6. The presence of lone pairs on the central atom contracts bond or inter pair angle to some extent. (The greater the number of lone pairs the greater will be the contraction of angles).

For example;

presence of a lone pair of electron reduces the angle from 109.5° to 107° in NH₃ while presence of two lone pairs of electrons reduces the angle from 109°.5 to 104.5° in water (H₂O).

🌿⚛7. The shape of molecule depends upon total no. of active electron pairs (bonding and lone pairs) or steric number. According to VSEPR theory, the active electron pairs arrangement around the central atom of a molecule determines the shape of a molecule i.e. the shape of molecule depends upon total number of active electron pairs or steric number.

It has been found that when total number of active electron pairs or steric number (bond pairs + lone pairs) is 2, 3 and 4, the spatial orientation of these electron pairs round the central atom is linear, trigonal planar and tetrahedral while the geometry of molecule will be linear, trigonal planar or bent, tetrahedral or pyramidal or angular respectively.

Of these shapes, the ones with no lone pairs are called the ideal shapes or idealized geometries. The five ideal shapes are: linear, trigonal planar, tetrahedral, trigonal bipramidal and octahedral. Each shape has a name and an idealized bond angle associated with it.

It is summarized as follows

Valence-Shell Electron-Pair Repulsion (VSEPR) Theory/VESPER Theory

🌈🔥Steps to Using VSEPR/Strategy or Sequence of steps in determining molecular geometry

1. Draw a Lewis structure for the ion or molecule in question.

2. Determine the number of electron groups around the central atom. Each lone pair of electrons counts as a single group. Each bond counts as a single group, even if it is a double or triple bond.

3. Find the corresponding electron geometry from the table.

4. Determine the number of lone pairs and the number of bonding pairs around the central atom, and use that to find the molecular geometry.

🌈🔥Limitations of VESPER Theory

1. It predicts and explains the shape of molecules but does not give reason for the formation of bonds.

(Lewis structures only tell the number and types of bonds between atoms, as they are limited to two dimensions. The VSEPR model predicts the 3-D shape of molecules and ions but is ineffective in providing any specific information regarding the bond length or the bond itself).

2. It is not applicable for single bond pair system i.e. diatomic molecules.

3. Complexes do not follow this theory.

[Complexes geometry are explained by crystal field theory (CFT) and ligand field theory (LFT)]

🌈🔥Prediction of Shapes of Different Molecules by VESPER Theory

🌈🔷 Summary: VSEPR Theory (MDCAT Quick Boost)

🔹 1. Core Idea

Atoms push electron pairs away from each other to reduce repulsion, guiding the molecule into the most stable shape.

🔹 2. What Repels What?

🟦 Lone pair ↔ Lone pair strongest repulsion

🟩 Lone pair ↔ Bond pair medium repulsion

🟨 Bond pair ↔ Bond pair weakest repulsion

More lone pairs means more distortion in geometry.

🔹 3. Geometry vs. Shape

🔥Geometry looks at all electron pairs.

🔥Shape looks at bonding pairs only (lone pairs shift angles).

🔹 4. Essential Patterns to Memorize

🟩 2 pairs → Linear → 180°

🟦 3 pairs → Trigonal planar → 120°

🟨 4 pairs → Tetrahedral → 109.5°

🟪 5 pairs → Trigonal bipyramidal → 90°, 120°

🟫 6 pairs → Octahedral → 90°

🔹 5. Lone Pair Effects (Exam Favourite)

🔥NH₃ angle < 109.5°

🔥H₂O angle < NH₃

🔥SO₂ bent, CO₂ linear

🔥PCl₅ different bond positions (axial + equatorial)

🔹 6. MDCAT Trigger Lines

🔥More lone pairs → smaller angle

🔥Double & triple bonds act as one region

🔥Shape depends on bonding pairs, not total pairs

🔥Lone pairs stay on the equatorial position first (for 5 regions)

🔷🔥 MDCAT/ECAT VSEPR Theory MCQs

1️⃣ Electron-pair geometry of CO₂ is:
🟥 A. Bent
🟦 B. Linear
🟩 C. Trigonal planar
🟨 D. Tetrahedral

2️⃣ Shape of SO₂ according to VSEPR:
🟥 A. Linear
🟦 B. Bent
🟩 C. Trigonal planar
🟨 D. Pyramidal

3️⃣ H₂O has bond angle less than 109.5° because:
🟥 A. Double bonds
🟦 B. Two lone pairs
🟩 C. More electronegativity
🟨 D. Four bonding pairs

4️⃣ NH₃ has which shape?
🟥 A. Bent
🟦 B. Trigonal planar
🟩 C. Trigonal pyramidal
🟨 D. Linear

5️⃣ Electron domains in BF₃:
🟥 A. Two
🟦 B. Three
🟩 C. Four
🟨 D. Five

6️⃣ Shape of PCl₅ according to VSEPR:
🟥 A. Tetrahedral
🟦 B. Trigonal bipyramidal
🟩 C. Octahedral
🟨 D. Square pyramidal

7️⃣ SF₆ has geometry:
🟥 A. Square planar
🟦 B. Trigonal bipyramidal
🟩 C. Octahedral
🟨 D. T-shaped

8️⃣ Lone pairs always prefer which position in 5-electron pair geometry?
🟥 A. Axial
🟦 B. Equatorial
🟩 C. Random
🟨 D. Peripheral

9️⃣ Shape of XeF₂ is:
🟥 A. Linear
🟦 B. Bent
🟩 C. See-saw
🟨 D. T-shaped

🔟 Electron-pair geometry of NH₄⁺:
🟥 A. Tetrahedral
🟦 B. Trigonal planar
🟩 C. Linear
🟨 D. Octahedral

1️⃣1️⃣ Shape of CH₄ is:
🟥 A. Trigonal pyramidal
🟦 B. Tetrahedral
🟩 C. Trigonal planar
🟨 D. Bent

1️⃣2️⃣ Which has maximum bond angle?
🟥 A. H₂O
🟦 B. NH₃
🟩 C. BF₃
🟨 D. CH₄

1️⃣3️⃣ NO₂⁻ has shape:
🟥 A. Bent
🟦 B. Linear
🟩 C. T-shaped
🟨 D. Pyramidal

1️⃣4️⃣ A molecule with 4 bonding pairs and 0 lone pairs is:
🟥 A. Linear
🟦 B. Tetrahedral
🟩 C. Bent
🟨 D. Square planar

1️⃣5️⃣ Regions of electron density in CO₃²⁻ are:
🟥 A. Four
🟦 B. Two
🟩 C. Three
🟨 D. Five

1️⃣6️⃣ Which molecule is see-saw shaped?
🟥 A. SF₄
🟦 B. SF₆
🟩 C. CO₂
🟨 D. NH₃

1️⃣7️⃣ Which has T-shaped geometry?
🟥 A. BrF₃
🟦 B. BF₃
🟩 C. NH₄⁺
🟨 D. CO₂

1️⃣8️⃣ A molecule with one lone pair and three bonding pairs is:
🟥 A. Linear
🟦 B. Bent
🟩 C. Trigonal pyramidal
🟨 D. Tetrahedral

1️⃣9️⃣ Central atom of XeF₄ is:
🟥 A. Trigonal planar
🟦 B. Octahedral
🟩 C. Square planar
🟨 D. Bent

2️⃣0️⃣ In VSEPR, double and triple bonds are counted as:
🟥 A. Two regions
🟦 B. Three regions
🟩 C. One region
🟨 D. Zero region

🎉 Answers
1️⃣ 🟦 B
2️⃣ 🟦 B
3️⃣ 🟦 B
4️⃣ 🟩 C
5️⃣ 🟦 B
6️⃣ 🟦 B
7️⃣ 🟩 C
8️⃣ 🟦 B
9️⃣ 🟥 A
🔟 🟥 A
1️⃣1️⃣ 🟦 B
1️⃣2️⃣ 🟩 C
1️⃣3️⃣ 🟥 A
1️⃣4️⃣ 🟦 B
1️⃣5️⃣ 🟩 C
1️⃣6️⃣ 🟥 A
1️⃣7️⃣ 🟥 A
1️⃣8️⃣ 🟩 C
1️⃣9️⃣ 🟩 C
2️⃣0️⃣ 🟩 C

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