VSEPR

Why do molecules have these shapes?

Through Building Molecules After Atoms , we already know that a molecule is essentially a structure formed by atoms “sharing electrons.” But new question immediately arise. Why do molecules adopt specific shapes? Why not some other way? For instance,

• Methane CH₄ —— Why is it a perfectly symmetrical “tetrahedron”?
• Water H₂O —— Why is it “bent”?

Behind this lies a very unified, highly “physical” rule.

The Core Rule: VSEPR

VSEPR (Valence Shell Electron Pair Repulsion theory) can be condensed into a single sentence:

Electron pairs repel each other and stay as far apart as possible.

There are only two key phrases here:

• Electron pairs (bonding pairs + lone pairs)
• Repulsion

You can imagine it as a bunch of electrons that absolutely hate each other, desperately trying to pull apart. And the shape of a molecule is simply the result of this “repulsion balance.”

Example 1: Methane CH₄

The structure of methane is very “clean”:

• Central atom: C
• 4 bonding electron pairs
• 0 lone pairs

So the question becomes:

How can 4 pairs of electrons arrange themselves to be the furthest apart?

The answer is:

Tetrahedron

• Bond angles are about 109.5°
• Completely symmetrical
• Minimum total repulsion

This is the most classic and foundational model of VSEPR. Many subsequent structures are essentially just “deformations” of this one.

Example 2: Water H₂O

On the surface, a water molecule seems quite different:

• Central atom: O
• 2 bonding electron pairs (connecting 2 H atoms)

But the crucial difference is that the oxygen atom also carries 2 lone pairs. Therefore, it actually still has 4 pairs of electrons around it. This means if we only look at the overall arrangement of “electron pairs,” water is still technically a tetrahedron.

However, the molecular shape we actually “see” only includes the atoms; it does not draw out the lone pairs. Consequently, the result becomes:

• The two lone pairs occupy two positions of the tetrahedron
• Furthermore, lone pairs exert stronger repulsion
• They “squeeze” the O–H bonds closer together

So it finally forms a Bent structure. The bond angle is about 104.5°, which is smaller than the ideal tetrahedral angle of 109.5°.

A Common Confusion

Many people get confused at this stage. If both CH₄ and H₂O are surrounded by 4 pairs of electrons, why are their shapes completely different?

Because there are actually two different levels of “shape” at play here:

• Electron Geometry

• Describes the arrangement of all electron pairs.

• Both CH₄ and H₂O are tetrahedral.

• Molecular Shape

• Describes only the positions of the atoms.

• CH₄ → tetrahedral

• H₂O → bent

What truly dictates all of this is not “what shape the atoms want to form,” but rather:

The repulsion between electrons.

In a sense, molecular geometry is essentially the equilibrium reached after a continuous game of tug-of-war between electrons.

From 2D Paper to the 3D World

Structural diagrams on paper look flat. But molecules in the real world are actually three-dimensional structures. For example, methane is not a “cross shape”

    H
    |
H – C – H
    |
    H

It truly exists in 3D space.

Now, we already have:

• Atoms
• Molecules
• Molecular geometry

A new question naturally emerges. What happens if these structures begin to “repeat indefinitely”? The answer is Crystal.

Further Reading

• Snow