Molecular Geometry — VSEPR Theory & Shapes

Molecular geometry is the three-dimensional shape of a molecule. In most intro chemistry courses, you predict that shape with VSEPR theory: electron domains around a central atom repel each other, so they arrange themselves as far apart as possible.

The fast way to think about it is this: count electron domains first, then separate electron-domain geometry from molecular geometry. Lone pairs are not atoms, but they still take up space, so they can change the shape and the bond angles.

How VSEPR Predicts Molecular Geometry

VSEPR stands for Valence Shell Electron Pair Repulsion. The model treats each region of electron density around the central atom as one domain that repels the others.

For basic VSEPR counting:

• one single bond counts as one domain
• one double bond counts as one domain
• one triple bond counts as one domain
• one lone pair counts as one domain

That last point is the one students usually miss. Two molecules can have the same total number of electron domains but different molecular geometries if one has lone pairs and the other does not.

Common Molecular Geometry Shapes

This table covers the shapes that appear most often in first-year chemistry:

Electron domains on central atom	Lone pairs on central atom	Molecular geometry	Common example
2	0	linear	$CO_2$
3	0	trigonal planar	$BF_3$
3	1	bent	$SO_2$
4	0	tetrahedral	$CH_4$
4	1	trigonal pyramidal	$NH_3$
4	2	bent	$H_2O$
5	0	trigonal bipyramidal	$PCl_5$
5	1	seesaw	$SF_4$
5	2	T-shaped	$ClF_3$
5	3	linear	$XeF_2$
6	0	octahedral	$SF_6$
6	1	square pyramidal	$BrF_5$
6	2	square planar	$XeF_4$

These are common VSEPR patterns, not a rule that fits every molecule perfectly. VSEPR works best as a first model for many main-group molecules and polyatomic ions.

Electron Geometry vs Molecular Geometry

This distinction causes a lot of wrong answers.

Electron-domain geometry describes the arrangement of all electron domains around the central atom, including lone pairs. Molecular geometry describes the arrangement of atoms only.

For example, $H_2O$ has four electron domains around oxygen, so its electron-domain geometry is tetrahedral. But only two of those domains are atoms bonded to oxygen, so its molecular geometry is bent.

Worked Example: Why Water Is Bent

Use $H_2O$ as the model case.

First, draw the Lewis structure. Oxygen is the central atom, bonded to two hydrogens, and it has two lone pairs.

Now count the electron domains around oxygen:

• two $O-H$ bonds
• two lone pairs

That makes four electron domains.

Four electron domains give a tetrahedral electron-domain geometry. If all four domains were bonding pairs, the molecular shape would also be tetrahedral, like $CH_4$. But in water, two of the domains are lone pairs.

So the molecular geometry is bent, not tetrahedral.

This also explains the bond angle. An ideal tetrahedral angle is about $109.5^\circ$, but the $H-O-H$ angle in water is smaller, about $104.5^\circ$. In the VSEPR model, lone pairs repel more strongly than bonding pairs, so they push the two $O-H$ bonds closer together.

That one example captures the main idea: the geometry depends on both the total number of electron domains and how many of them are lone pairs.

How To Determine Molecular Geometry Step By Step

Use this sequence for most introductory problems:

1. Draw a reasonable Lewis structure.
2. Identify the central atom.
3. Count electron domains around that central atom.
4. Assign the electron-domain geometry from the domain count.
5. Ignore lone pairs when naming the molecular geometry, but do not ignore them when reasoning about repulsion and bond angles.

If the Lewis structure is wrong, the geometry will usually be wrong too. VSEPR starts with structure, not with memorizing a shape chart by itself.

Common Mistakes In VSEPR Problems

Counting A Multiple Bond As More Than One Domain

In VSEPR, a double bond or triple bond still counts as one electron domain around the central atom. The electron density is distributed differently, but for the basic geometry count it is still one region.

Mixing Up Electron Geometry And Molecular Geometry

This is why students often call water tetrahedral. Water is tetrahedral only in its electron-domain geometry. Its molecular geometry is bent.

Ignoring Lone Pairs

Lone pairs are not visible as atoms in the final shape name, but they strongly affect the geometry and often reduce bond angles relative to the ideal value.

Treating VSEPR As Exact In Every Case

VSEPR gives a useful first prediction, especially for many main-group species. It is less reliable when bonding is more complex, such as in many transition-metal compounds or cases where a more detailed orbital picture matters.

When Molecular Geometry Matters

Molecular geometry is used to predict bond angles, polarity, and reactivity trends. It also helps explain why molecules with the same atoms can behave differently if their shapes differ.

For example, shape helps you reason about whether bond dipoles cancel, whether a molecule is likely to be polar, and how atoms are positioned for intermolecular interactions or reactions.

Try A Similar Molecule

Try your own version with $NH_3$ or $CO_2$. Draw the Lewis structure, count electron domains around the central atom, and name both the electron-domain geometry and the molecular geometry.

If you want one more step after that, compare the shape you found with the molecule's polarity and ask whether the bond dipoles cancel.