General chemistry

Molecular geometry (VSEPR), orbital hybridization

Predicting the shape of a molecule from the repulsion between electron pairs around the central atom.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionIntuition: electron pairs keep their distance

Electron pairs around a central atom repel each other, so they spread as far apart as possible. Counting bonding pairs (X) and lone pairs (E) around the central atom A gives the AXE notation; the arrangement of all the pairs sets the molecular shape. This is the VSEPR model (valence-shell electron-pair repulsion).

Pick a shape and turn the lone pairs (orange lobes) on and off. Lone pairs take more room than bonding pairs, which squeezes the bond angles.

SchoolSchool level: from pair counts to shapes

AXE type, shape, ideal angle and an example
AXE	Shape	Angle	Example
AX₂	linear	180°	$\ce{CO2}$
AX₃	trigonal planar	120°	$\ce{BF3}$
AX₂E	bent	< 120°	$\ce{SO2}$
AX₄	tetrahedral	109.5°	$\ce{CH4}$
AX₃E	trigonal pyramidal	≈ 107°	$\ce{NH3}$
AX₂E₂	bent	≈ 104.5°	$\ce{H2O}$
AX₅	trigonal bipyramidal	90°, 120°	$\ce{PCl5}$
AX₄E	seesaw	< 90°, < 120°	$\ce{SF4}$
AX₆	octahedral	90°	$\ce{SF6}$
AX₄E₂	square planar	90°	$\ce{XeF4}$

Example: The shape of $\ce{NH3}$

Predict the shape and approximate bond angle of ammonia.

Solution

Nitrogen has 5 valence electrons: 3 pairs bond to H and 1 lone pair remains. That is AX₃E: 4 electron pairs (tetrahedral arrangement), so the shape is trigonal pyramidal with H–N–H about 107°.

UndergraduateUndergraduate: hybridization

Valence-bond theory describes the same shapes with hybrid orbitals: two, three, four, five and six electron pairs correspond to $sp$ , $sp^2$ , $sp^3$ , $sp^3d$ and $sp^3d^2$ hybridization. Hybridization describes a shape once it is known; it does not by itself predict it.

Switch between

sp

sp^2

and

sp^3

: the number of lobes and the angles between them match the AX

_n

shapes above (180°, 120°, 109.5°).