VSEPR Theory

If you have ever tied balloons together at a birthday party, you already understand VSEPR theory intuitively. Tie two balloons at their knots - they point in opposite directions (180 degrees apart). Tie three together - they spread into a triangle (120 degrees). Tie four - they form a tetrahedron (109.5 degrees). The balloons naturally push apart as far as possible to minimize crowding.

Electron groups around a central atom do the exact same thing.

What Is VSEPR?

VSEPR stands for Valence Shell Electron Pair Repulsion. The core idea is simple: electron groups around a central atom are all negatively charged, so they repel each other. To minimize that repulsion, they arrange themselves as far apart as possible in three-dimensional space.

This arrangement determines the shape of the molecule - which in turn determines its polarity, reactivity, and biological function.

Steric Number: Counting Electron Groups

Before you can predict geometry, you need to count the electron groups around the central atom. This count is called the steric number.

An electron group is any of the following:

• A single bond (counts as 1 group)
• A double bond (counts as 1 group)
• A triple bond (counts as 1 group)
• A lone pair (counts as 1 group)

The critical rule: each multiple bond counts as only ONE electron group, no matter how many electron pairs it contains. A double bond has 4 electrons and a triple bond has 6, but each occupies one region of space around the central atom.

The Five Base Electron Geometries

The steric number directly tells you the electron geometry - the arrangement of all electron groups (both bonds and lone pairs) around the central atom.

| Steric Number | Electron Geometry | Ideal Bond Angle(s) | Example |
|:---:|:---:|:---:|:---:|
| 2 | Linear | 180° | CO₂ |
| 3 | Trigonal planar | 120° | BF₃ |
| 4 | Tetrahedral | 109.5° | CH₄ |
| 5 | Trigonal bipyramidal | 90° and 120° | PCl₅ |
| 6 | Octahedral | 90° | SF₆ |

These five geometries are the foundation of everything in VSEPR. Memorize them cold.

Electron Geometry vs. Molecular Geometry

Here is where many students get tripped up. Electron geometry describes the arrangement of all electron groups, including lone pairs. Molecular geometry describes the arrangement of only the atoms - what you would actually “see” if you could photograph the molecule.

When there are no lone pairs, the two geometries are identical. But when lone pairs are present, the molecular geometry is a subset of the electron geometry - because lone pairs are invisible in the molecular shape.

We will explore this distinction in full detail in the next section. For now, understand that the electron geometry is determined by the steric number, and the molecular geometry depends on how many of those groups are bonds versus lone pairs.

Steric Number 2: Linear Geometry

With only two electron groups, the groups point in opposite directions to maximize their distance. The bond angle is exactly 180 degrees.

Examples: CO₂ (two double bonds), HCN (one triple bond + one single bond), BeCl₂ (two single bonds).

All three molecules are perfectly linear. Note that CO₂ has double bonds and HCN has a triple bond, but each multiple bond still counts as one electron group.

Steric Number 3: Trigonal Planar Geometry

Three electron groups arrange in a flat triangle with 120-degree angles between them. All atoms lie in the same plane.

Examples: BF₃ (three single bonds, no lone pairs), formaldehyde H₂CO (two single bonds + one double bond), NO₃⁻ (resonance hybrid with three equivalent bonds).

Steric Number 4: Tetrahedral Geometry

Four electron groups point toward the corners of a tetrahedron, with bond angles of 109.5 degrees. This is a three-dimensional shape - it cannot be drawn accurately in two dimensions.

Examples: CH₄ (four single bonds), NH₄⁺ (four single bonds), CCl₄ (four single bonds).

The tetrahedral geometry is the most common on the MCAT because carbon forms four bonds in the vast majority of organic molecules.

Steric Number 5: Trigonal Bipyramidal Geometry

Five electron groups create a more complex shape with two distinct positions:

• Equatorial positions (3 groups in the “belt”): 120 degrees apart from each other, 90 degrees from the axial positions.
• Axial positions (2 groups on “top” and “bottom”): 180 degrees from each other, 90 degrees from the equatorial positions.

Example: PCl₅ (five single bonds to phosphorus).

This geometry is unique because not all positions are equivalent - the equatorial and axial positions have different bond angles. This matters when lone pairs are present, because lone pairs preferentially occupy equatorial positions (where there is more room).

Steric Number 6: Octahedral Geometry

Six electron groups point toward the corners of an octahedron, with all bond angles at 90 degrees. Unlike the trigonal bipyramid, all six positions are equivalent.

Example: SF₆ (six single bonds to sulfur).

An octahedron looks like two square-based pyramids glued at their bases. All positions have the same relationship to the central atom.

Comprehensive VSEPR Reference Table

This table summarizes the electron geometries, molecular geometries, and bond angles for all steric numbers. The molecular geometry column shows what happens when some electron groups are lone pairs rather than bonds.

| Steric Number | Lone Pairs | Bonding Groups | Electron Geometry | Molecular Geometry | Approx. Bond Angle | Example |
|:---:|:---:|:---:|:---:|:---:|:---:|:---:|
| 2 | 0 | 2 | Linear | Linear | 180° | CO₂ |
| 3 | 0 | 3 | Trigonal planar | Trigonal planar | 120° | BF₃ |
| 3 | 1 | 2 | Trigonal planar | Bent | ~118° | SO₂ |
| 4 | 0 | 4 | Tetrahedral | Tetrahedral | 109.5° | CH₄ |
| 4 | 1 | 3 | Tetrahedral | Trigonal pyramidal | ~107° | NH₃ |
| 4 | 2 | 2 | Tetrahedral | Bent | ~104.5° | H₂O |
| 5 | 0 | 5 | Trigonal bipyramidal | Trigonal bipyramidal | 90°, 120° | PCl₅ |
| 5 | 1 | 4 | Trigonal bipyramidal | Seesaw | ~90°, ~120° | SF₄ |
| 5 | 2 | 3 | Trigonal bipyramidal | T-shaped | ~90° | ClF₃ |
| 5 | 3 | 2 | Trigonal bipyramidal | Linear | 180° | XeF₂ |
| 6 | 0 | 6 | Octahedral | Octahedral | 90° | SF₆ |
| 6 | 1 | 5 | Octahedral | Square pyramidal | ~90° | BrF₅ |
| 6 | 2 | 4 | Octahedral | Square planar | 90° | XeF₄ |

How to Predict Geometry: A Quick Algorithm

1. Draw the Lewis structure.
2. Count the steric number (bonds + lone pairs on the central atom).
3. Determine the electron geometry from the steric number.
4. Determine the molecular geometry by noting how many of those groups are lone pairs.

This four-step process works for every VSEPR problem on the MCAT. Practice it until it becomes automatic.

Why VSEPR Matters for the MCAT

VSEPR is not just an abstract exercise in geometry. Molecular shape determines:

• Polarity: A molecule’s shape determines whether bond dipoles cancel or add up. CO₂ is linear and nonpolar; H₂O is bent and very polar.

• Reactivity: The shape of an enzyme’s active site must match the shape of the substrate (lock-and-key model). Molecular geometry is central to biochemistry.

• Physical properties: Boiling points, melting points, and solubility all depend on intermolecular forces, which depend on molecular polarity, which depends on shape.

Understanding VSEPR connects bonding to virtually every other topic on the MCAT.