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Lesson 1 · 30 min

Valence-shell electron-pair repulsion (VSEPR)

Electron pairs repel and spread as far apart as they can — that single idea predicts the shapes of most small molecules, and explains why the bond angles of CH₄, NH₃ and H₂O shrink one after another.

In this lesson

Count the electron domains around a central atom from its Lewis structure and write the AXₙEₘ label
Predict the electron-domain geometry from the number of domains, then the molecular shape from where the lone pairs go
Explain the bond-angle trend CH₄ > NH₃ > H₂O with “lone pairs take up more room”
State the limits of VSEPR — for example why H₂S has a bond angle near 90°

Methane, ammonia and water look unrelated, yet each central atom is surrounded by exactly four pairs of valence electrons. Methane’s H–C–H angle is 109.5°, ammonia’s H–N–H 106.7°, water’s H–O–H 104.5° — each smaller than the last. One period down, hydrogen sulfide’s H–S–H is only 92.1°. One simple idea explains the first three numbers, and the fourth tells us where that idea runs out.

Predict first

A central atom has four electron domains: three bonding pairs and one lone pair (as in $\ce{NH3}$ ). What shape do the atoms form?

Pick an answer and lock it in before you look. A wrong guess teaches more than no guess.

Trigonal planar Tetrahedral Trigonal pyramidal T-shaped

Electron domains and shapes

The heart of VSEPR is one sentence: the electron domains around a central atom repel one another and get as far apart as possible. A domain can be a single, double or triple bond (a multiple bond counts as one domain) or a lone pair.

Count the domains from the Lewis structure to fix the electron-domain geometry: 2 give linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral. Then ask how many are lone pairs — the molecular shape is named from the atoms alone. Chemists write $\mathrm{AX}_n\mathrm{E}_m$ : A is the central atom, X an atom bonded to it, E a lone pair. Water is $\mathrm{AX_2E_2}$ : tetrahedral domains, bent molecule.

Common AXₙEₘ types
Domains	Type	Shape	Example
2	$\mathrm{AX_2}$	linear, 180°	$\ce{CO2}$ , $\ce{BeCl2}$
3	$\mathrm{AX_3}$	trigonal planar, 120°	$\ce{BF3}$
3	$\mathrm{AX_2E}$	bent, < 120°	$\ce{SO2}$ , $\ce{O3}$
4	$\mathrm{AX_4}$	tetrahedral, 109.5°	$\ce{CH4}$
4	$\mathrm{AX_3E}$	trigonal pyramidal	$\ce{NH3}$
4	$\mathrm{AX_2E_2}$	bent	$\ce{H2O}$
5	$\mathrm{AX_5}$	trigonal bipyramidal, 90° and 120°	$\ce{PCl5}$
5	$\mathrm{AX_4E}$	seesaw	$\ce{SF4}$
5	$\mathrm{AX_3E_2}$	T-shaped	$\ce{ClF3}$
5	$\mathrm{AX_2E_3}$	linear	$\ce{XeF2}$
6	$\mathrm{AX_6}$	octahedral, 90°	$\ce{SF6}$
6	$\mathrm{AX_5E}$	square pyramidal	$\ce{BrF5}$
6	$\mathrm{AX_4E_2}$	square planar	$\ce{XeF4}$

Lone pairs take up more room

If all domains were alike, NH₃ and H₂O would also have 109.5° angles. They are smaller because a lone pair is held by only one nucleus, so its cloud is fatter and sits closer to the central atom, pushing harder on the other domains. The repulsions rank: lone pair–lone pair > lone pair–bond pair > bond pair–bond pair. The more lone pairs, the more tightly the bonding pairs are squeezed: CH₄ (none) 109.5°, NH₃ (one) 106.7°, H₂O (two) 104.5°.

The same idea places lone pairs in five-domain molecules: they always take equatorial positions of the trigonal bipyramid, where they have only two neighbours at 90° instead of three. Hence SF₄ is a seesaw, ClF₃ T-shaped and XeF₂ linear.

Valence-shell electron-pair repulsion · 3D

VSEPR shape explorer

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Ammonia

Show lone pairs Show bond dipoles Click atoms to measure

VSEPR notation: $\mathrm{AX_{3}E_{1}}$
Electron-domain geometry: Tetrahedral
Molecular shape: Trigonal pyramidal
Measured angle: 106.7°
Bond-dipole sum: polar

Switch between CH₄, NH₃ and H₂O and read the measured angles; tick “Click atoms to measure” and measure them yourself. Then look at PH₃ and H₂S — their angles fall far below what VSEPR expects.

Hydrides with four electron domains: measured bond angles
Molecule	Lone pairs	Angle
$\ce{CH4}$	0	109.5°
$\ce{NH3}$	1	106.7°
$\ce{H2O}$	2	104.5°
$\ce{PH3}$	1	93.5°
$\ce{H2S}$	2	92.1°

Source: Gas-phase experimental geometries, NIST CCCBDB (as stored in the site’s molecules.ts).

Worked example

Predict the shape of ClF₃

Use VSEPR to predict the shape of $\ce{ClF3}$ .

Concept check

What is the shape of $\ce{XeF4}$ ?

Tetrahedral Seesaw Square planar Square pyramidal

Try the calculation

How many lone pairs sit on the central Xe atom of $\ce{XeF2}$ ?

Key takeaways

Electron domains (single bonds, multiple bonds or lone pairs) repel and spread out: 2, 3, 4, 5 and 6 domains give linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral arrangements.
Molecular shape is named from the atoms alone; the AXₙEₘ label records both bonded atoms and lone pairs.
Lone pairs take more room than bonding pairs: CH₄ 109.5° > NH₃ 106.7° > H₂O 104.5°; with five domains, lone pairs go equatorial.
VSEPR gives trends only: heavier hydrides such as H₂S (92.1°) and transition-metal compounds lie beyond it.

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Data and references

R. J. Gillespie, R. S. Nyholm, “Inorganic stereochemistry”, Q. Rev. Chem. Soc. 11, 339 (1957); R. J. Gillespie, I. Hargittai, The VSEPR Model of Molecular Geometry (Allyn & Bacon, 1991).
NIST Computational Chemistry Comparison and Benchmark Database (CCCBDB): experimental bond lengths and angles.
C. E. Housecroft, A. G. Sharpe, Inorganic Chemistry, 5th ed., Ch. 2 (VSEPR and its limitations).