TL;DR:
Regions of electrons around a central atom repel each other and spread as far apart as possible. Count the regions to get the layout.
Two regions give a line (180°), three a triangle (120°), four a tetrahedron (109.5°).
Lone pairs take up space but are not atoms, so 4 regions with 1 lone pair give a pyramid, and with 2 lone pairs a bent shape.
A Lewis structure shows which atoms are bonded, but it is flat. Real molecules are three-dimensional, and their shape decides how they behave: whether they are polar, how they pack and how they react. A simple rule predicts the shape: electron clouds repel, so they get as far apart as they can.
The rule: valence shell electron pair repulsion
VSEPR stands for valence shell electron pair repulsion. Around a central atom, each bond and each lone pair is a domain, a region of electron density. A double or triple bond counts as one domain, since all its electrons sit in the same region between the two atoms.
Domains carry negative charge, so they repel. The arrangement that keeps them farthest apart sets the electron-domain geometry. The molecular shape is then what you get by looking only at the atoms, ignoring lone pairs.
Domains | Arrangement | Angle |
|---|---|---|
2 | Linear | 180° |
3 | Trigonal planar | 120° |
4 | Tetrahedral | 109.5° |
Worked example: methane is tetrahedral
Lewis structure of CH₄: carbon is central, with four single bonds to hydrogen and no lone pairs. Electrons:
4 + 4 × 1 = 8, matching four bonds.Count domains on carbon: 4 bonds and 0 lone pairs, so 4 domains.
Four domains spread out into a tetrahedron, with the hydrogens at the corners.
Bond angle: 109.5° between any two C-H bonds.
Where does 109.5° come from? Put the carbon at the centre of a cube and use four of the cube's corners for the hydrogens, no two adjacent. The angle between two bond directions satisfies cos θ = −1/3, so θ = 109.47°. That rounds to 109.5°.
A flat square, with 90° angles, would crowd the hydrogens closer together. The tetrahedron is the best way to place four points on a sphere.
What lone pairs do
A lone pair is a domain too, so it takes a corner of the arrangement. But the shape names describe only the atoms, so the lone pair is invisible in the name.
Molecule | Bonds + lone pairs | Shape | Approximate angle |
|---|---|---|---|
CH₄ | 4 + 0 | Tetrahedral | 109.5° |
NH₃ | 3 + 1 | Trigonal pyramidal | about 107° |
H₂O | 2 + 2 | Bent | about 104.5° |
CO₂ | 2 + 0 | Linear | 180° |
BF₃ | 3 + 0 | Trigonal planar | 120° |
Ammonia: Lewis structure with nitrogen central, 3 bonds and 1 lone pair, 4 domains. The domains point to the corners of a tetrahedron, but only three have atoms, so the molecule is a pyramid. Water: oxygen has 2 bonds and 2 lone pairs, 4 domains, and two corners are empty of atoms, so it is bent.
The angles in NH₃ and H₂O are slightly smaller than 109.5° because lone pairs, held only by one nucleus, are more spread out and repel the bonding pairs more strongly. Each lone pair squeezes the bond angles a bit. The exact values are measured, so quote them as approximate.
One more case shows the whole method. Carbon dioxide, CO₂, has the structure O=C=O. Carbon has two double bonds and no lone pairs, so 2 domains, and the shape is linear at 180°. Compare sulfur dioxide, SO₂, where sulfur has two bonded oxygens and one lone pair. That is 3 domains in a triangle with one corner empty, so the shape is bent with an angle a little under 120°.
The pattern is to go from formula to Lewis structure, from Lewis structure to domain count, from domain count to the layout, and from the layout to the shape by deleting the lone pairs. Writing each stage on its own line keeps the steps from blurring together.
How to check your answer
Draw the Lewis structure first, with the right total of electrons. See drawing Lewis structures step by step.
Count domains, not atoms. Lone pairs count, and a double bond counts once.
Domains minus lone pairs gives the number of bonded atoms. It must match the formula.
Angles. Lone pair shapes should have angles a little under the ideal value.
Shape is not the whole story: VSEPR predicts the shape of simple molecules around a central atom. It does not cover every molecule, especially large or metal-containing ones, so treat it as a first estimate.
Mistakes that cost marks
Counting a double bond as two domains.
Naming the shape from the domain count and forgetting the lone pairs.
Saying water is linear because it has three atoms.
Forgetting that the central atom's lone pairs, not those on the outer atoms, determine the shape.
Quoting 109.5° for water or ammonia without noting the lone-pair squeeze.
Where you will use it next
Shape decides polarity: CO₂ is linear and its two polar bonds cancel, while bent water does not cancel. To see where the electrons come from in the first place, read electron configurations and the periodic table.
Frequently asked questions
Why is water bent when CO₂ is linear?
Carbon in CO₂ has two domains and no lone pairs, so it is linear. Oxygen in water has four domains, two of them lone pairs, so the atoms form a bent shape.
Is the tetrahedral angle exactly 109.5°?
The ideal value is 109.47°, which is usually rounded to 109.5°. Real molecules with the same atoms on all four corners, like methane, match it.
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