Difficulty: Introductory to Intermediate | Prerequisites: Lewis structures (dot diagrams), basic understanding of valence electrons and chemical bonding.
This topic sits at the heart of general chemistry: once you can draw a Lewis structure, VSEPR theory tells you the actual three-dimensional shape of the molecule. That shape determines everything from boiling points to biological activity. The lab uses ChemBio3D Ultra to visualise and measure molecular geometries computationally, but the conceptual foundation, predicting shape from electron domains, is what you need for the exam. If you are not yet comfortable drawing Lewis structures or counting valence electrons, revisit those notes first.
VSEPR theory predicts a molecule's 3D shape by counting the electron domains (bonding pairs and lone pairs) around a central atom. The domains spread as far apart as possible to minimise repulsion, giving rise to characteristic geometries: linear (2 domains), trigonal planar (3), tetrahedral (4), and octahedral (6). Knowing the geometry lets you predict bond angles, polarity, and physical properties.
VSEPR (Valence Shell Electron Pair Repulsion)
A model that predicts the geometry of a molecule based on the idea that electron domains around a central atom will arrange themselves to be as far apart as possible.
In simple terms, electrons repel each other, so the groups of electrons around an atom spread out to get maximum distance from one another, and that arrangement is the molecule's shape.
Electron domain (electron group)
Any region of electron density around a central atom. A single bond, a double bond, a triple bond, and a lone pair each count as one electron domain.
Think of it as: each "clump" of electrons, whether it is a bond or a lone pair, counts as one domain.
Bonding pair
A pair of electrons shared between two atoms in a covalent bond.
In simple terms, the glue holding two atoms together.
Lone pair (non-bonding pair)
A pair of valence electrons on an atom that is not shared with another atom. Lone pairs still occupy space and influence molecular shape.
Think of it as: invisible arms on the central atom that push the visible bonds around.
Lewis structure (electron-dot structure)
A diagram showing all valence electrons in a molecule as dots or lines (bonds). The starting point for predicting geometry.
Bond angle
The angle formed between two bonds that share a common atom. Measured in degrees.
Bond length
The distance between the nuclei of two bonded atoms, typically measured in angstroms (A) or picometres (pm). Shorter bonds are generally stronger.
Molecular geometry (molecular shape)
The three-dimensional arrangement of only the atoms in a molecule (lone pairs are excluded from the name of the shape, though they influence it).
Electron-domain geometry (electron geometry)
The three-dimensional arrangement of all electron domains, both bonding and non-bonding, around the central atom.
In simple terms, this is the "full picture" including lone pairs, whereas molecular geometry is the shape you see with atoms only.
Structural optimisation (energy minimisation)
A computational process that adjusts a molecule's geometry to find the arrangement with the lowest potential energy, i.e. the most stable structure.
Think of it as: the computer nudges atoms around until the molecule is as relaxed as possible.
The central idea: electron domains around a central atom repel one another and arrange themselves to maximise the distance between them.
Count every electron domain around the central atom. A single bond = 1 domain. A double bond = 1 domain. A triple bond = 1 domain. A lone pair = 1 domain.
The total number of electron domains gives you the electron-domain geometry. The arrangement of atoms only (ignoring lone pairs) gives you the molecular geometry.
Lone pairs occupy more space than bonding pairs. They compress the bond angles below the "ideal" values. For example, the ideal tetrahedral angle is 109.5 degrees, but in water (two lone pairs) the H-O-H angle is roughly 104.5 degrees.
2 electron domains: electron-domain geometry is linear. If both are bonding, the molecular shape is linear with a bond angle of 180 degrees. Example: CO2.
3 electron domains: electron-domain geometry is trigonal planar. If all three are bonding, the molecular shape is trigonal planar with bond angles of 120 degrees. Example: BF3. If one is a lone pair, the shape becomes bent (roughly 117 degrees or less).
4 electron domains: electron-domain geometry is tetrahedral. If all four are bonding, the molecular shape is tetrahedral with bond angles of 109.5 degrees. Example: CH4. With one lone pair, the shape is trigonal pyramidal (e.g. NH3, roughly 107 degrees). With two lone pairs, the shape is bent (e.g. H2O, roughly 104.5 degrees).
5 electron domains: electron-domain geometry is trigonal bipyramidal. Various molecular shapes depending on lone-pair count (seesaw, T-shaped, linear).
6 electron domains: electron-domain geometry is octahedral. If all six are bonding, the molecular shape is octahedral with bond angles of 90 degrees. Example: SF6.
Higher bond order means a shorter, stronger bond. A triple bond is shorter than a double bond, which is shorter than a single bond between the same pair of elements.
Larger atoms form longer bonds. The C=O double bond in CO2 (approximately 1.16 A) is shorter than the C=S double bond in CS2 (approximately 1.55 A) because sulphur is a larger atom than oxygen.
Replacing one atom in a molecule with a larger or smaller atom changes bond lengths but may not change the overall geometry if the electron-domain count stays the same.
CO2 (Carbon Dioxide)
Lewis structure: O=C=O (double bond on each side, no lone pairs on carbon).
Electron domains on C: 2 (two double bonds).
Electron-domain geometry: linear.
Molecular shape: linear.
Bond angle: 180 degrees.
C=O bond length: approximately 1.16 A.
Both C=O bonds are identical in length and strength.
BF3 (Boron Trifluoride)
Lewis structure: boron at the centre with three single bonds to fluorine. Boron has only 6 electrons (an exception to the octet rule).
Electron domains on B: 3 (three bonding pairs, no lone pairs).
Electron-domain geometry: trigonal planar.
Molecular shape: trigonal planar.
Bond angles: 120 degrees.
B-F bond length: approximately 1.31 A.
All three B-F bonds are equivalent.
CH4 (Methane)
Lewis structure: carbon at the centre with four single bonds to hydrogen.
Electron domains on C: 4 (four bonding pairs, no lone pairs).
Electron-domain geometry: tetrahedral.
Molecular shape: tetrahedral.
Bond angles: 109.5 degrees.
C-H bond length: approximately 1.09 A.
All four C-H bonds are identical.
H2O (Water)
Lewis structure: oxygen at the centre with two single bonds to hydrogen and two lone pairs.
Electron domains on O: 4 (two bonding pairs + two lone pairs).
Electron-domain geometry: tetrahedral.
Molecular shape: bent (V-shaped).
H-O-H bond angle: approximately 104.5 degrees (compressed from 109.5 by lone-pair repulsion).
O-H bond length: approximately 0.96 A.
The lone pairs are not visible in the molecular shape name but they are why water is bent, not linear.
SF6 (Sulphur Hexafluoride)
Lewis structure: sulphur at the centre with six single bonds to fluorine. Sulphur is an expanded-octet atom (12 electrons around it).
Electron domains on S: 6 (six bonding pairs, no lone pairs).
Electron-domain geometry: octahedral.
Molecular shape: octahedral.
Bond angles: 90 degrees (between adjacent F-S-F bonds).
S-F bond length: approximately 1.56 A.
All six S-F bonds are equivalent.
Molecule | Electron Domains | Electron Geometry | Molecular Shape | Ideal Bond Angle | Approximate Bond Length |
|---|---|---|---|---|---|
CO2 | 2 | Linear | Linear | 180 degrees | C=O: 1.16 A |
BF3 | 3 | Trigonal planar | Trigonal planar | 120 degrees | B-F: 1.31 A |
CH4 | 4 | Tetrahedral | Tetrahedral | 109.5 degrees | C-H: 1.09 A |
H2O | 4 (2 bonding + 2 lone) | Tetrahedral | Bent | ~104.5 degrees | O-H: 0.96 A |
SF6 | 6 | Octahedral | Octahedral | 90 degrees | S-F: 1.56 A |
Key relationship: Bond order up, bond length down, bond strength up. A double bond is shorter and stronger than a single bond between the same atoms; a triple bond is shorter and stronger still.
Molecular geometry determines whether a molecule is polar or nonpolar, which in turn controls solubility, boiling point, and how molecules interact in biological systems. CO2 is linear and nonpolar, which is why it is a gas at room temperature rather than dissolving readily in nonpolar solvents. Water is bent and polar, which is the reason it is such an effective solvent for ionic and polar substances.
Drug design relies heavily on molecular shape. A drug molecule must fit precisely into a receptor, and even a small change in bond angle or length can make it ineffective. The same logic applies to enzyme-substrate interactions in biochemistry.
Students often think a double bond counts as two electron domains. It does not. A double bond (or triple bond) is one electron domain, regardless of how many electron pairs it contains.
Students frequently confuse electron-domain geometry with molecular geometry. Electron-domain geometry includes lone pairs in the arrangement. Molecular geometry describes only where the atoms are. For water, the electron-domain geometry is tetrahedral, but the molecular shape is bent.
Students sometimes assume that all molecules with four atoms around a central atom are tetrahedral. They are not. If one of those "atoms" is actually a lone pair, the shape changes (e.g. trigonal pyramidal for NH3).
Students often forget that lone pairs compress bond angles. The bond angle in water (about 104.5 degrees) is smaller than the ideal tetrahedral angle (109.5 degrees) because lone pairs repel bonding pairs more strongly than bonding pairs repel each other.
⚠️ You will almost certainly be asked to predict the shape of a molecule from its Lewis structure. Practise the sequence: draw Lewis structure, count electron domains, name electron-domain geometry, name molecular shape.
⚠️ Questions comparing molecules (e.g. CO2 vs CS2, CH4 vs CH3Cl) are common. Know that replacing an atom changes bond lengths but may preserve the overall geometry if the electron-domain count is unchanged.
⚠️ Be ready to explain why water is bent and not linear. This is a classic exam question.
⚠️ Know the difference between electron-domain geometry and molecular geometry. Exam questions often require both.
⚠️ Expect questions on how lone pairs affect bond angles. The pattern: more lone pairs on the central atom means smaller bond angles.
True or false: A double bond counts as two electron domains. (False, it counts as one.)
Fill in the blank: The molecular shape of a molecule with four electron domains and two lone pairs is __________. (Bent.)
True or false: SF6 has an expanded octet on sulphur. (True, sulphur has 12 electrons around it.)
Fill in the blank: Lone pairs compress bond angles because they exert __________ repulsion than bonding pairs. (Greater / more.)
True or false: CO2 and CS2 have different molecular shapes. (False, both are linear.)
Q: Draw the Lewis structure of BF3 and predict its molecular geometry. Why does boron not follow the octet rule here?
A: BF3 has boron at the centre with three single bonds to fluorine, giving boron only 6 valence electrons. Boron is electron-deficient and is a well-known exception to the octet rule. With three bonding domains and no lone pairs, the geometry is trigonal planar with 120-degree bond angles.
Q: Explain why water has a bond angle of approximately 104.5 degrees rather than the ideal tetrahedral angle of 109.5 degrees.
A: Water has four electron domains (two bonding pairs and two lone pairs). The electron-domain geometry is tetrahedral, but the two lone pairs repel the bonding pairs more strongly than the bonding pairs repel each other. This extra repulsion compresses the H-O-H angle from 109.5 degrees down to about 104.5 degrees.
Q: CO2 and CS2 are both linear molecules. How do their bond lengths differ, and why?
A: Both have two double bonds to the central carbon atom, so both are linear with 180-degree bond angles. However, sulphur is a larger atom than oxygen, so the C=S bond (approximately 1.55 A) is longer than the C=O bond (approximately 1.16 A).
Q: What is the molecular shape of SF6, and why can sulphur accommodate six bonds?
A: SF6 is octahedral with 90-degree F-S-F bond angles. Sulphur is in Period 3 and has access to empty 3d orbitals, which allows it to expand its octet and form six bonds.
Q: If you replace one hydrogen in CH4 with a chlorine atom to form CH3Cl, does the overall geometry change? What about the bond angles?
A: The overall geometry remains roughly tetrahedral (four bonding domains, no lone pairs). However, the bond angles are no longer all 109.5 degrees because chlorine is larger and more electronegative than hydrogen. The Cl-C-H angles and H-C-H angles will differ slightly from the ideal value.
This connects directly to Lewis structures (Molecular Modeling I), which are the prerequisite for predicting geometry. If you cannot draw the Lewis structure correctly, the VSEPR prediction will be wrong.
Molecular polarity builds on this material: once you know the shape, you can determine whether bond dipoles cancel (nonpolar, like CO2) or reinforce (polar, like H2O). That feeds into intermolecular forces, solubility, and phase behaviour later in the course.
Orbital hybridisation (sp, sp2, sp3, sp3d, sp3d2) is the quantum-mechanical explanation for these geometries. The number of electron domains maps directly to the hybridisation: 2 domains = sp, 3 = sp2, 4 = sp3, and so on.
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