Molecular Shapes (VSEPR)

What is it?

VSEPR stands for valence-shell electron-pair repulsion. It predicts the 3D shape of a molecule from one simple idea: the regions of electrons around a central atom repel each other and spread out to be as far apart as possible.

Each region of electrons around the central atom is called an electron domain. A domain is:

a lone pair, or
a bond to another atom: single, double or triple (a multiple bond counts as one domain).

Key idea

Count the electron domains around the central atom to get the electron-domain geometry. Then look only at where the atoms are to name the molecular shape. Lone pairs affect the shape but are not part of its name.

Why does it matter?

Shape controls properties. Whether a molecule is polar, and so how it dissolves and how high it boils, depends on its shape.
Shape controls biology. Enzymes, drugs and smell receptors work because molecules have particular shapes that fit together.
It’s quick. With a Lewis structure, you can predict shapes and bond angles in seconds.

How does it work?

1. Steps

Draw the Lewis structure.
Count the electron domains on the central atom (bonds + lone pairs; multiple bonds count once).
Find the electron-domain geometry from the number of domains.
Name the molecular shape from the positions of the atoms.

2. The shapes

Domains	Lone pairs	Molecular shape	Bond angle	Example
2	0	linear	180°	$\ce{CO2}$
3	0	trigonal planar	120°	$\ce{BF3}$
3	1	bent	slightly less than 120°	$\ce{SO2}$
4	0	tetrahedral	109.5°	$\ce{CH4}$
4	1	trigonal pyramidal	about 107°	$\ce{NH3}$
4	2	bent	about 104.5°	$\ce{H2O}$
5	0	trigonal bipyramidal	90° and 120°	$\ce{PCl5}$
6	0	octahedral	90°	$\ce{SF6}$

3. Why lone pairs squeeze bond angles

A lone pair is held by only one nucleus, so it sits closer to the central atom, takes up more space and repels more strongly than a bonding pair. The order of repulsion is:

lone pair–lone pair > lone pair–bond pair > bond pair–bond pair

That’s why the angle drops from 109.5° in $\ce{CH4}$ to about 107° in $\ce{NH3}$ (one lone pair) and about 104.5° in $\ce{H2O}$ (two lone pairs).

4. Shape and polarity

A molecule is polar if its bond polarities do not cancel out.

$\ce{CO2}$ has two polar C=O bonds, but they point in opposite directions (linear), so they cancel: nonpolar.
$\ce{H2O}$ has two polar O–H bonds in a bent shape, so they don’t cancel: polar.
$\ce{CCl4}$ (tetrahedral, four identical bonds): nonpolar. $\ce{CHCl3}$ (one bond different): polar.

Think of it like this

Tie four balloons together at their knots. They automatically point to the corners of a tetrahedron, because each balloon pushes the others away. Two balloons sit in a line; three spread out flat in a triangle.

More precisely

With 5 or 6 domains, lone pairs give further shapes. With 5 domains, lone pairs go in the flat “equator” positions, giving seesaw ( $\ce{SF4}$ , 1 lone pair), T-shaped ( $\ce{ClF3}$ , 2) and linear ( $\ce{XeF2}$ , 3). With 6 domains, you get square pyramidal ( $\ce{BrF5}$ , 1) and square planar ( $\ce{XeF4}$ , 2).

Visualise it

Eight molecular shapes. Linear CO2, 180°, 2 domains. Trigonal planar BF3, 120°, 3 domains. Bent SO2, about 119°, 3 domains with 1 lone pair. Tetrahedral CH4, 109.5°, 4 domains. Trigonal pyramidal NH3, about 107°, 4 domains with 1 lone pair. Bent H2O, about 104.5°, 4 domains with 2 lone pairs. Trigonal bipyramidal PCl5, 90° and 120°, 5 domains. Octahedral SF6, 90°, 6 domains. Wedges point towards the viewer, dashed bonds point away, and lobes show lone pairs. — Common VSEPR shapes. Multiple bonds count as one domain; outer-atom lone pairs are not shown.

Worked example

Worked example: Ammonia, NH₃

Lewis structure: N bonded to three H, with one lone pair on N.
Domains on N: 3 bonds + 1 lone pair = 4, so the electron-domain geometry is tetrahedral.
Atoms occupy only three corners, so the shape is trigonal pyramidal, with H–N–H about 107°.
The N–H bond polarities don’t cancel in this shape, so $\ce{NH3}$ is polar.

Worked example: Formaldehyde, CH₂O

Lewis structure: C in the centre, single bonds to two H and a double bond to O; no lone pairs on C.
Domains on C: 3 (the C=O double bond counts once).
Shape: trigonal planar, angles about 120°.

Common mistake

Common mistake: Counting a double bond as two domains

A double or triple bond points in one direction, so it counts as one electron domain. $\ce{CO2}$ has 2 domains on carbon, not 4.

Common mistake: Naming the shape from the electron domains

Water has 4 domains (tetrahedral electron geometry) but its molecular shape is bent, because only the atoms are counted when naming the shape.

Common mistake: Assuming polar bonds mean a polar molecule

$\ce{CO2}$ and $\ce{CCl4}$ have polar bonds but are nonpolar molecules, because their symmetry makes the bond polarities cancel.

Notation note

In 3D drawings, a solid wedge points towards you and a dashed bond points away; ordinary lines lie in the plane of the page.
Some books write “AXₘEₙ” notation: A = central atom, X = bonded atoms, E = lone pairs. For example, water is AX₂E₂.

Remember this

Electron domains (bonds + lone pairs, multiple bonds count once) spread out as far as possible.
2 linear 180°; 3 trigonal planar 120°; 4 tetrahedral 109.5°; 5 trigonal bipyramidal; 6 octahedral.
Lone pairs repel more and squeeze bond angles: CH₄ 109.5°, NH₃ about 107°, H₂O about 104.5°.
Polar molecule = polar bonds that do not cancel by symmetry.

Practise this topic with flashcards and a quiz at chemistryclarity.com/chemistry/molecular-shapes/