VSEPR Calculator
Predict molecular geometry using VSEPR theory.
Enter the bonding domains and lone pairs to get the molecular shape, bond angle, and AXnEm notation.
VSEPR stands for Valence Shell Electron Pair Repulsion. The idea is simple: electron pairs around a central atom repel each other and spread out as far apart as possible. The arrangement of those electron domains determines the molecular geometry.
Bonding domains and lone pairs both count as electron domains for determining the electron geometry (the three-dimensional arrangement of all domains). But the molecular geometry only describes the positions of atoms, not lone pairs. A molecule with four electron domains and one lone pair has tetrahedral electron geometry but trigonal pyramidal molecular geometry.
Lone pairs repel more strongly than bonding pairs because they are held closer to the nucleus and are more spread out. This causes bond angles to compress below the ideal values. Water has two bonding pairs and two lone pairs on oxygen, giving tetrahedral electron geometry but with bond angles of 104.5 degrees instead of the theoretical 109.5 degrees.
The AXnEm notation encodes the geometry compactly: A is the central atom, X is a bonding domain, E is a lone pair. AX4 is tetrahedral methane. AX3E1 is trigonal pyramidal ammonia. AX2E2 is bent water.
Common molecular geometries: AX2: linear (180 degrees). CO2, HCN. AX3: trigonal planar (120 degrees). BF3, SO3. AX2E: bent (~120 degrees). SO2, O3. AX4: tetrahedral (109.5 degrees). CH4, SiCl4. AX3E: trigonal pyramidal (~107 degrees). NH3, PCl3. AX2E2: bent (~104.5 degrees). H2O, H2S. AX5: trigonal bipyramidal. PCl5. AX4E: see-saw. SF4. AX3E2: T-shaped. ClF3. AX2E3: linear (lone pairs equatorial). XeF2. AX6: octahedral. SF6. AX5E: square pyramidal. BrF5. AX4E2: square planar. XeF4.
VSEPR works well for main-group elements. Transition metal complexes follow different rules based on crystal field and ligand field theory.
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