What Is Molecular Geometry? (VSEPR Chemistry Guide)
Molecular geometry is the three-dimensional arrangement of atoms in a molecule. The VSEPR model predicts that shape by counting electron domains around a central atom: bonding regions and lone pairs. Count the domains to find the electron-domain geometry, then consider only the atom positions to name the molecular geometry. The result is a useful prediction, not an exact measurement.
If a Lewis structure looks like a flat set of lines, it can be hard to picture the molecule it represents. VSEPR gives you a practical way to turn that drawing into a three-dimensional shape. The key is to keep two ideas separate: the arrangement of all electron domains and the arrangement of atoms.
What molecular geometry means
Molecular geometry describes where a molecule’s atoms sit in three-dimensional space. It names the shape made by the atoms, not the full arrangement of electrons. Lone pairs help determine that shape, but they are not counted as atoms when you name it.
This distinction prevents a common mistake. In ammonia, NH₃, the nitrogen has four electron domains: three bonding regions and one lone pair. The electron-domain geometry is tetrahedral, but the molecular geometry is trigonal pyramidal because the shape name describes the positions of the three hydrogen atoms and the nitrogen.
VSEPR stands for Valence-Shell Electron-Pair Repulsion. It is a model for predicting shape based on the idea that electron domains around a central atom arrange themselves to reduce repulsion. The model is especially useful for simple molecules and is a standard starting point in chemistry courses.
How VSEPR counts electron domains
An electron domain is a region of electron density around the central atom. For VSEPR counting, each bond to another atom counts as one domain, whether it is a single, double, or triple bond. Each lone pair on the central atom also counts as one domain.
Do not count every line in a double or triple bond as a separate domain. A double bond has more electron density than a single bond, but both count as one domain for this basic shape prediction. Count only domains around the central atom, not lone pairs on outer atoms.
The letters in a VSEPR formula make the count easier to show:
- A is the central atom.
- X is an atom bonded to the central atom.
- E is a lone pair on the central atom.
For example, AX₃E means a central atom with three bonded atoms and one lone pair. It has four electron domains in total.
A reliable step-by-step VSEPR method
A reliable VSEPR prediction starts with a sound Lewis structure. From there, count the domains around the central atom, identify the electron-domain geometry, and name the molecular geometry. Check the predicted angles last, because lone pairs and bond types can shift real angles from ideal values.
Use these steps for each structure:
- Draw the Lewis structure. Count the valence electrons, connect the atoms, and place lone pairs. Check formal charges when needed. A formal charge is an accounting tool that helps assess whether the electron arrangement is reasonable.
- Find the central atom. Hydrogen is usually an outer atom because it forms one bond. For other atoms, the Lewis structure and formula help identify the atom connected to the others.
- Count domains around the central atom. Count each bond as one domain and each central-atom lone pair as one. A multiple bond still counts as one domain.
- Name both geometries. Use the total domain count for electron-domain geometry. Then ignore lone pairs when naming the molecular geometry, while remembering that lone pairs affect the positions of the bonded atoms.
- Estimate angles and check the result. Compare the shape with its ideal angle pattern. Treat the angle as an estimate, not a guaranteed experimental value.
I find it useful to write both names beside a structure, such as “electron-domain geometry: tetrahedral; molecular geometry: trigonal pyramidal.” This simple check makes it less likely that you will call NH₃ tetrahedral or confuse its shape with the arrangement of all four domains.
Electron-domain geometries and common shapes
The number of electron domains gives the basic electron-domain geometry. Molecular geometry depends on how many of those domains are bonds and how many are lone pairs. The table lists common predictions and ideal angle patterns; real bond angles can differ.
| Domains | Electron-domain geometry | Example VSEPR formula | Molecular geometry | Ideal angle pattern |
|---|---|---|---|---|
| 2 | Linear | AX₂ | Linear | 180° |
| 3 | Trigonal planar | AX₃ | Trigonal planar | 120° |
| 3 | Trigonal planar | AX₂E | Bent | Less than 120° |
| 4 | Tetrahedral | AX₄ | Tetrahedral | About 109.5° |
| 4 | Tetrahedral | AX₃E | Trigonal pyramidal | About 109.5° ideal; often smaller |
| 4 | Tetrahedral | AX₂E₂ | Bent | Smaller than 109.5° |
| 5 | Trigonal bipyramidal | AX₅ | Trigonal bipyramidal | 90°, 120°, and 180° |
| 6 | Octahedral | AX₆ | Octahedral | 90° and 180° |
The angles in the table describe idealized arrangements. Lone pairs often repel nearby bonding regions more strongly than bonding regions repel one another. This can compress bond angles. The size and identity of atoms, bond types, and the surrounding molecular environment can also affect the measured shape.
For five domains, positions are not all equivalent: a trigonal bipyramidal arrangement has three equatorial positions and two axial positions. A lone pair often favors an equatorial position because this reduces close interactions. For six domains, an AX₄E₂ arrangement is commonly described as square planar.
Worked VSEPR examples
Worked examples show how the domain count changes the shape name. Each example uses the same sequence: identify the central atom, count its bonds and lone pairs, find the electron-domain geometry, and then name the arrangement of atoms.
Water, H₂O
Oxygen is the central atom. It has two O–H bonds and two lone pairs, so there are four domains. Four domains give a tetrahedral electron-domain geometry, but the two hydrogen atoms form a bent molecular geometry. The H–O–H angle is about 104.5°, below the ideal tetrahedral angle of about 109.5°.
Ammonia, NH₃
Nitrogen has three N–H bonds and one lone pair, for four domains. Its electron-domain geometry is tetrahedral, while its molecular geometry is trigonal pyramidal. The bond angle is about 107°, somewhat less than the ideal tetrahedral angle.
Carbon dioxide, CO₂
Carbon is the central atom, with a double bond to each oxygen. Each double bond counts as one domain, so carbon has two domains and no lone pairs. The electron-domain and molecular geometries are both linear, with an ideal angle of 180°.
Sulfur tetrafluoride, SF₄
Sulfur has four S–F bonding regions and one lone pair, making five domains. The electron-domain geometry is trigonal bipyramidal. The lone pair occupies an equatorial position in the usual VSEPR model, leaving the atoms in a seesaw molecular geometry.
Using geometry to reason about polarity
Molecular shape helps you decide whether bond dipoles may cancel. A bond dipole is a small separation of charge across a bond. To assess molecular polarity, consider both the bond dipoles and the three-dimensional arrangement of the atoms.
A symmetrical molecule with identical outer atoms can have bond dipoles that cancel. Carbon dioxide is linear, so its two C=O bond dipoles point in opposite directions and cancel. Water is bent, so its O–H bond dipoles do not cancel. Its shape supports a polar molecule.
Geometry alone is not enough to determine polarity. You also need to consider whether the bonds are polar and whether their dipoles cancel. VSEPR helps establish the shape, but a complete polarity prediction uses both shape and bond information.
What VSEPR can and cannot tell you
VSEPR is a practical model for predicting common molecular shapes and approximate bond angles. It is not a precise bond-angle calculator or a full quantum-mechanical account of molecular structure. For exact angles or complex molecules, measured data or more advanced models may be needed.
A good workflow is to use VSEPR to make a first prediction, then compare it with reliable experimental or computational data when precision matters. Differences from ideal angles do not automatically mean that your domain count is wrong. They may reflect the atoms, bonds, or molecular environment.
Key takeaway: Count bonds and lone pairs around the central atom to find electron-domain geometry. Then name molecular geometry from atom positions only. Keep the two labels distinct, and treat ideal angles as guides rather than exact values.
Frequently asked questions
What is molecular geometry in simple terms?
Molecular geometry is the three-dimensional shape formed by a molecule’s atoms. It describes where the atoms sit relative to one another, while the electron-domain geometry also accounts for lone pairs on the central atom.
What does VSEPR stand for?
VSEPR stands for Valence-Shell Electron-Pair Repulsion. It is a model that predicts molecular shapes by considering how electron domains around a central atom arrange themselves to reduce repulsion.
How do I count electron domains?
Count each bond to the central atom as one domain, including double and triple bonds. Then count each lone pair on that atom as one more domain. Do not count lone pairs on outer atoms for the central atom’s geometry.
What is the difference between electron-domain geometry and molecular geometry?
Electron-domain geometry includes both bonding regions and lone pairs around the central atom. Molecular geometry describes only the positions of atoms. Lone pairs affect the shape, but they do not appear as atoms in its name.
Why is NH₃ trigonal pyramidal, not tetrahedral?
Nitrogen in NH₃ has three bonded hydrogen atoms and one lone pair. All four domains have a tetrahedral arrangement, but the three atoms form a trigonal pyramid. Tetrahedral describes the electron-domain geometry.
What shape is a molecule with two electron domains?
With two domains around the central atom, the electron-domain geometry is linear. If both domains are bonds and there are no lone pairs on that atom, the molecular geometry is also linear, with an ideal angle of 180°.
Does a double bond count as two electron domains?
No. In basic VSEPR counting, a double bond counts as one domain, just like a single or triple bond. Its electron density can affect bond angles, but it does not add extra domains to the count.
Are VSEPR bond angles exact?
No. Values such as 109.5° for a tetrahedral arrangement are idealized predictions. Lone pairs, bond types, atom identity, and the molecular environment can change actual angles. Use measurements when an exact value matters.
(This article was written by one of our staff writers, Nathan Sterling. Visit our Meet the Team page.)
