Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the 3D geometry of molecules based on the idea that electron pairs around a central atom repel each other and arrange themselves to minimize repulsion. Both bonding pairs (shared electrons in covalent bonds) and lone pairs (non-bonding electrons) contribute to the molecular shape. The theory was developed by Ronald Gillespie and Ronald Nyholm in 1957 and remains one of the most powerful tools for predicting molecular geometry.

The AX_mE_n notation is used to classify VSEPR shapes:
• A = Central atom
• X = Number of bonding pairs (atoms bonded to the central atom)
• E = Number of lone electron pairs on the central atom
For example, H₂O has AX₂E₂ (2 bonds, 2 lone pairs), giving it a bent shape. NH₃ is AX₃E₁ (3 bonds, 1 lone pair) with a trigonal pyramidal geometry.

Lone pairs occupy more space than bonding pairs because they are attracted to only one nucleus (the central atom), making their electron cloud more diffuse. This creates stronger repulsion: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair. As a result, lone pairs push bonding pairs closer together, compressing bond angles. For example, the ideal tetrahedral angle is 109.5°, but in H₂O (AX₂E₂), the H–O–H angle is reduced to ~104.5° due to two lone pairs.

The steric number is the sum of bonding pairs and lone pairs around the central atom: Steric Number = m + n (from AX_mE_n). It determines the basic electron geometry:
• SN=2 → Linear (180°)
• SN=3 → Trigonal Planar (120°)
• SN=4 → Tetrahedral (109.5°)
• SN=5 → Trigonal Bipyramidal (90°, 120°)
• SN=6 → Octahedral (90°)
The actual molecular shape then depends on how many of those positions are occupied by lone pairs vs. bonding pairs.

While VSEPR theory is remarkably effective, it has limitations:
• It does not explain magnetic properties of molecules
• It cannot predict bond lengths accurately
• Transition metal complexes often deviate from VSEPR predictions due to d-orbital involvement
• Highly delocalized systems (like benzene) are better described by molecular orbital theory
• It's a qualitative model — for precise angles, quantum mechanical calculations (like DFT) are needed
Despite these limitations, VSEPR remains the go-to method for quick, accurate geometry predictions in main-group chemistry.

Follow these steps:
1. Draw the Lewis structure of the molecule.
2. Count the number of bonding domains (single, double, or triple bonds each count as one domain) = m.
3. Count the number of lone pairs on the central atom = n.
4. Determine the steric number (m + n) to find the electron geometry.
5. Use the AX_mE_n notation to identify the molecular shape. Use our interactive 3D viewer above to explore each shape and understand how lone pairs affect the final geometry!

SF₄ has AX₄E₁ — steric number 5 (trigonal bipyramidal electron geometry). The lone pair occupies an equatorial position because equatorial positions have fewer 90° interactions (2 neighbors at 90°) compared to axial positions (3 neighbors at 90°). This minimizes repulsion. The result is a "seesaw" (or disphenoidal) molecular shape with the lone pair in the equatorial plane, two fluorines axial, and two equatorial, with bond angles slightly less than 90° and 120°.

Electron geometry describes the spatial arrangement of all electron domains (both bonding pairs and lone pairs) around the central atom. Molecular geometry describes the arrangement of only the atoms (bonding pairs), ignoring lone pairs. For example, water (H₂O) has a tetrahedral electron geometry (4 domains: 2 bonds + 2 lone pairs) but a bent molecular geometry (only the 2 O–H bonds define the shape). The molecular geometry is what we actually observe experimentally.