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The VSEPR Theory Calculator predicts molecular geometry and electron geometry from the number of bonding pairs and lone pairs around a central atom. Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs around a central atom arrange themselves to minimize repulsion, determining the three-dimensional shape of the molecule. The steric number (sum of bonding and lone pairs) determines the electron geometry, while the actual molecular shape depends on which positions are occupied by bonding pairs versus lone pairs. This model, developed by Ronald Gillespie and Ronald Nyholm, is one of the most widely used tools in general chemistry for predicting molecular shapes without complex calculations. Enter the number of bonding pairs and lone pairs on the central atom to determine the geometry.
VSEPR theory works in three steps:
Step 1: Calculate the Steric Number
Steric Number = Bonding Pairs + Lone Pairs
This determines the electron geometry:
Step 2: Determine Molecular Shape
The molecular shape depends on both the steric number and the number of lone pairs:
Step 3: Predict Bond Angles
Lone pairs occupy more space than bonding pairs, compressing bond angles slightly below the ideal values. For example, water (SN=4, LP=2) has bond angles of 104.5 degrees rather than the ideal 109.5 degrees.
The steric number tells you the electron geometry. The molecular shape is determined by the positions of atoms only (ignoring lone pairs). Lone pairs reduce observed bond angles because they exert stronger repulsion than bonding pairs. Molecules with no lone pairs have ideal bond angles matching the electron geometry. The electron geometry code and molecular shape code are numerical identifiers that map to the shape names listed in the How It Works section above.
Inputs
Results
Water has 2 bonding pairs and 2 lone pairs, giving steric number 4. Electron geometry is tetrahedral (code 4), but molecular shape is bent (SN=4, LP=2). The ideal tetrahedral angle of 109.5 degrees is compressed to 104.5 degrees by the lone pairs.
Inputs
Results
Methane has 4 bonding pairs and 0 lone pairs, giving steric number 4. Both electron geometry and molecular shape are tetrahedral (code 40) with ideal bond angles of 109.5 degrees.
Electron geometry considers all electron pairs (bonding + lone) around the central atom. Molecular shape (or molecular geometry) describes only the arrangement of atoms, ignoring lone pairs. For example, NH3 has tetrahedral electron geometry but trigonal pyramidal molecular shape.
Draw the Lewis structure first. Each single bond counts as 1 bonding pair; double and triple bonds also count as 1 bonding pair (electron domain) for VSEPR purposes. Lone pairs are non-bonding electron pairs shown as dots on the central atom. Total valence electrons minus bonding electrons divided by 2 gives lone pairs.
No. In VSEPR theory, each bond (single, double, or triple) counts as one electron domain. A double bond occupies the same spatial position as a single bond. However, multiple bonds do exert slightly more repulsion than single bonds.
Lone pairs are held closer to the central atom than bonding pairs and spread over a larger volume. This means lone pair-lone pair and lone pair-bonding pair repulsions are greater than bonding pair-bonding pair repulsions, pushing bonding pairs closer together.
VSEPR works well for main group elements but is less reliable for transition metal complexes, where crystal field theory or ligand field theory is more appropriate. It also has limitations for molecules with significant ionic character or very large central atoms.
The codes encode the geometry numerically. The electron geometry code equals the steric number (2=linear, 3=trigonal planar, 4=tetrahedral, 5=trigonal bipyramidal, 6=octahedral). The molecular shape code combines steric number and lone pairs (e.g., 42 = SN=4 with 2 lone pairs = bent).
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