About the VSEPR Molecular Geometry Calculator
This VSEPR calculator predicts the three-dimensional shape of a molecule or polyatomic ion from the electron domains around its central atom. Enter the number of atoms bonded to the central atom and its lone pairs, count them from valence electrons, or pick a common example, and it returns the AXₘEₙ notation, the electron-domain geometry, the molecular geometry, ideal bond angles, the usual hybridization label and whether the shape is polar.
It is designed for general chemistry students learning valence shell electron pair repulsion (VSEPR) theory and for anyone checking a Lewis-structure answer. A double or triple bond counts as a single electron domain, and lone pairs repel more strongly than bonding pairs, which is why NH₃ and H₂O have angles smaller than 109.5°.
The electron-count mode uses the shortcut SN = ½(V + M − charge), where V is the central atom’s valence electrons and M the number of monovalent atoms such as H or halogens; doubly bonded O or S atoms add a domain but no electrons. It works for typical main-group molecules; transition-metal complexes and molecules with several central atoms need a full Lewis structure. Polarity assumes all outer atoms are identical.
How to use the vsepr molecular geometry calculator
- 1Draw or picture the Lewis structure and identify the central atom.
- 2Count the atoms bonded to it and its lone pairs — or switch to valence-electron mode and enter the counts.
- 3Read the molecular geometry, electron geometry and ideal bond angles.
- 4Use the table to compare shapes with the same steric number.
Formula and method
VSEPR theory says electron domains around a central atom — bonds and lone pairs — arrange themselves as far apart as possible. The steric number (bonded atoms plus lone pairs) fixes the electron-domain geometry: 2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral and 7 pentagonal bipyramidal. A multiple bond counts as one domain.
The molecular geometry describes only the atom positions, so lone pairs change the name of the shape: four domains with one lone pair is trigonal pyramidal, with two it is bent. Because lone pairs repel more strongly than bonding pairs, angles shrink below the ideal values. In the electron-count shortcut, monovalent atoms each share one electron with the central atom, doubly bonded O or S share none, and a negative charge adds an electron.
- A
- Central atom
- X
- Number of atoms bonded to the central atom
- E
- Number of lone pairs on the central atom
- V
- Valence electrons of the central atom
- M, D
- Monovalent outer atoms (H, halogens) and doubly bonded O/S atoms
Worked examples
Ammonia-type AX₃E
Three bonded atoms and one lone pair give a steric number of 4, so the electrons are arranged tetrahedrally. With one position taken by a lone pair the atoms form a trigonal pyramid, with angles squeezed to about 107°.
Sulfate ion SO₄²⁻ from electron counting
Sulfur has 6 valence electrons, oxygens contribute none and the 2− charge adds 2, so SN = (6 + 0 + 2) ÷ 2 = 4. All four domains are bonds to oxygen, so SO₄²⁻ is tetrahedral at 109.5°.
Xenon tetrafluoride from electron counting
Xenon has 8 valence electrons and four fluorines add 4, giving SN = 12 ÷ 2 = 6. Four bonds leave two lone pairs, which sit opposite each other in the octahedron, so XeF₄ is square planar with 90° F–Xe–F angles.
Chlorine trifluoride preset
ClF₃ has three Cl–F bonds and two lone pairs. The lone pairs take two equatorial positions of the trigonal bipyramid, leaving the atoms in a T shape with F–Cl–F angles just under 90°.
Frequently asked questions
What is the difference between electron geometry and molecular geometry?+
Electron geometry describes how all electron domains, including lone pairs, are arranged. Molecular geometry describes only where the atoms are. Water has a tetrahedral electron geometry but a bent molecular geometry because two domains are lone pairs.
Do double and triple bonds count as one electron domain?+
Yes. In VSEPR a single, double or triple bond to one atom counts as a single domain, because all the shared electrons lie in the same region between the two nuclei. CO₂ therefore has two domains and is linear.
Why is the bond angle in water 104.5° and not 109.5°?+
Lone pairs are held closer to the central atom and spread out more than bonding pairs, so they push the bonds together. Each lone pair reduces the angle further: CH₄ 109.5°, NH₃ about 107°, H₂O about 104.5°.
How do I find the number of lone pairs on the central atom?+
Subtract the electrons used in bonding from the central atom’s valence electrons (adjusted for charge) and divide by two. For simple molecules the shortcut SN = ½(V + M − charge) gives the total domains; lone pairs are SN minus bonded atoms.
Is a molecule with polar bonds always polar?+
No. Symmetric shapes such as linear, trigonal planar, tetrahedral, square planar and octahedral cancel identical bond dipoles, so CO₂, BF₃, CH₄ and SF₆ are nonpolar. Lone pairs or different outer atoms usually make the molecule polar.