Source: Lecture notes / slide deck
Tags: VSEPR, valence shell electron pair repulsion, molecular geometry, tetrahedral, bent, bond angle, Lewis structure, lone pairs, ligands
Difficulty: Introductory to Intermediate. Prerequisites: basic understanding of Lewis dot structures, electron configurations, and the concept of valence electrons.
VSEPR theory is one of the foundational models for predicting the three-dimensional shapes of molecules. It sits right at the bridge between drawing flat Lewis structures and understanding how molecules actually look in space, which matters for everything from reactivity to polarity. You need a solid grip on valence electrons and Lewis structures before this will click. If you skipped that material, go back and revise it first, because VSEPR builds directly on top of it.
VSEPR (Valence Shell Electron Pair Repulsion) predicts molecular shapes by assuming that electron groups around a central atom push each other as far apart as possible. The number of bonding groups (ligands) and lone pairs determines the geometry: four bonding groups with no lone pairs gives a tetrahedral shape (like methane), while two bonding groups with two lone pairs gives a bent shape (like water). Both have bond angles of roughly 109.5 degrees.
VSEPR (Valence Shell Electron Pair Repulsion)
A model for predicting the 3D geometry of a molecule based on the repulsion between electron pairs (both bonding and non-bonding) around a central atom. In simple terms, electron groups want to be as far apart from each other as possible, and that spacing determines the shape.
Ligand
An atom or group bonded to the central atom. Think of it as anything attached to the middle atom in your Lewis structure. The number of ligands is part of what determines the molecular shape.
Tetrahedral geometry
The shape adopted by a molecule with four bonding groups and no lone pairs around the central atom. The bond angles are approximately 109.5 degrees. Methane (CH4) is the classic example.
Bent geometry
The shape adopted by a molecule when lone pairs on the central atom compress the bonding pairs closer together. Water (H2O) is bent because oxygen has two lone pairs pushing the two O-H bonds downward. The bond angle is still approximately 109.5 degrees (though in practice slightly less).
Lone pair
A pair of valence electrons on an atom that is not involved in bonding. Lone pairs occupy more space than bonding pairs and therefore exert greater repulsive force, which alters the molecular geometry.
Bond angle
The angle formed between two bonds originating from the same central atom. VSEPR predicts specific bond angles for each geometry: 109.5 degrees for tetrahedral, 120 degrees for trigonal planar, 180 degrees for linear.
Electron pairs in the valence shell of a central atom repel each other
They arrange themselves to maximise the distance between them
Both bonding pairs and lone pairs count as "electron groups"
The number of electron groups determines the electron geometry; the number of bonding groups (minus lone pairs) determines the molecular shape
Carbon has 4 valence electrons; each hydrogen has 1
Total valence electrons: C (4) + 4H (4 x 1) = 8 valence electrons
Carbon forms 4 C-H bonds, satisfying the octet rule (8 electrons around carbon)
4 ligands, 0 lone pairs on the central atom = tetrahedral shape
Bond angle: approximately 109.5 degrees
Drawing conventions for 3D structures:
Solid line = bond in the plane of the page
Solid wedge = bond pointing out of the plane (towards you)
Dashed wedge = bond pointing into the plane (away from you)
Oxygen has 6 valence electrons; each hydrogen has 1
Oxygen forms 2 O-H bonds and retains 2 lone pairs
2 bonding groups + 2 lone pairs = 4 electron groups total
Electron geometry is still tetrahedral, but molecular shape is bent because lone pairs are not visible in the molecular shape
Bond angle: approximately 109.5 degrees (in practice, slightly less than 109.5 because lone pairs compress the bonding pairs)
The bent shape is caused specifically by the presence of lone pairs on the central atom
Lone pairs take up more space than bonding pairs
They push bonding pairs closer together, reducing the bond angle
The same electron-group count can produce different molecular shapes depending on how many groups are lone pairs versus bonding pairs
Molecule | Central Atom | Bonding Groups | Lone Pairs | Electron Groups | Molecular Shape | Bond Angle |
|---|---|---|---|---|---|---|
CH4 (methane) | C | 4 | 0 | 4 | Tetrahedral | ~109.5° |
H2O (water) | O | 2 | 2 | 4 | Bent | ~109.5° (slightly less) |
Key relationship: the total number of electron groups (bonding + lone pairs) determines the electron geometry. The molecular shape is then determined by which of those groups are bonding pairs versus lone pairs.
Use solid lines for bonds in the plane
Use solid (filled) wedges for bonds coming out of the plane towards the viewer
Use dashed (hashed) wedges for bonds going behind the plane away from the viewer
The combination of these three line types allows you to represent a 3D tetrahedral arrangement on a 2D page
Molecular geometry determines physical properties like polarity, boiling point, and solubility. Water's bent shape is precisely why it is a polar molecule, which in turn is why it is such an effective solvent for ionic and polar compounds. Understanding VSEPR is a prerequisite for predicting how molecules interact in reactions, in biological systems, and in material design.
Students often confuse electron geometry with molecular geometry. They are not the same thing. Electron geometry counts all electron groups (including lone pairs); molecular geometry only describes where the atoms are.
Students sometimes assume that water is linear because it has only two bonds. It is not. The two lone pairs on oxygen make it bent.
Students frequently forget that lone pairs occupy more space than bonding pairs, which is why they compress bond angles below the ideal values.
Students may think the octet rule and VSEPR are the same concept. The octet rule tells you how many bonds and lone pairs an atom has; VSEPR then tells you how those groups arrange themselves in space.
Be prepared to identify the molecular shape from a Lewis structure. This is a very common exam question.
Know the difference between electron geometry and molecular geometry.
Memorise the bond angles for tetrahedral (109.5°), trigonal planar (120°), and linear (180°).
Be able to draw 3D representations using wedge-and-dash notation.
Understand how lone pairs change the molecular shape even when the electron geometry stays the same.
True or false: A molecule with 4 electron groups around the central atom is always tetrahedral in molecular shape. (False, it could be bent or trigonal pyramidal depending on lone pairs.)
Fill in the blank: The bond angle in a tetrahedral molecule is approximately ______ degrees. (109.5)
True or false: Lone pairs occupy less space than bonding pairs. (False, they occupy more space.)
Fill in the blank: Water has a ______ molecular shape because oxygen has two lone pairs. (Bent)
True or false: A solid wedge in a 3D drawing means the bond points away from the viewer. (False, it points towards the viewer.)
Q: What is the molecular geometry of CH4, and why?
A: Tetrahedral. Carbon has four bonding groups (four C-H bonds) and no lone pairs, so all four electron groups are equivalent and arrange themselves at 109.5 degree angles.
Q: Why is water (H2O) bent rather than linear?
A: Oxygen has two bonding pairs (the two O-H bonds) and two lone pairs. The lone pairs push the bonding pairs closer together, producing a bent shape rather than a straight line.
Q: What is the approximate bond angle in both CH4 and H2O, and why are they similar?
A: Both are approximately 109.5 degrees because both central atoms have four electron groups arranged tetrahedrally. In water, the angle is slightly less than 109.5 because lone pairs compress the bonding pairs.
Q: How do you represent a 3D molecular structure on paper?
A: Use three types of bonds. A solid line for bonds in the plane of the page, a solid (filled) wedge for bonds coming towards you, and a dashed wedge for bonds going away from you.
Q: Does the octet rule tell you the shape of a molecule?
A: No. The octet rule tells you how many bonds and lone pairs an atom has. VSEPR theory then uses that information to predict the 3D shape.
This material connects directly to orbital hybridisation (sp3, sp2, sp), which provides the quantum-mechanical explanation for why molecules adopt these geometries. It also links to polarity and dipole moments, because molecular shape determines whether bond dipoles cancel or reinforce each other. Later in organic chemistry, molecular geometry becomes critical for understanding stereochemistry, chirality, and reaction mechanisms.
VSEPR, valence shell electron pair repulsion, molecular geometry, electron geometry, tetrahedral, bent shape, bond angle, 109.5 degrees, lone pairs, bonding pairs, ligands, Lewis structure, Lewis dot structure, wedge-dash notation, 3D molecular shape, methane CH4, water H2O, octet rule, central atom, electron groups, steric number, molecular shape prediction, organic chemistry geometry