Why Molecules Have Shapes

VSEPR chart and shape finder

Set the bonded atoms and lone pairs on a central atom, or tap a shape in the chart below.

The finder shows the shape for the counts you set, here ammonia's: three bonded atoms and one lone pair, a trigonal pyramid. With Measured angles on, the model takes the measured geometry of a real example. Every one of the thirteen common shapes is in the chart; tap one to load it.

How the finder works

A shape needs two numbers, both read from the central atom of the Lewis structure: the number of atoms bonded to it (a double or triple bond still counts once) and the number of lone pairs on it. Their sum, the steric number, fixes the electron geometry; where the lone pairs sit fixes the molecular geometry[1]. The finder spreads the domains by letting them repel on a sphere, then places lone pairs by the usual rules: equatorial in a trigonal bipyramid, opposite each other in an octahedron.

DomainsLone pairsAXEElectron geometryMolecular geometryIdeal anglesExample (measured)
20AX2linearlinear180°CO2 180°
30AX3trigonal planartrigonal planar120°BF3 120°
31AX2Etrigonal planarbent120°SO2 119.5°
40AX4tetrahedraltetrahedral109.5°CH4 109.5°
41AX3Etetrahedraltrigonal pyramidal109.5°NH3 106.7°
42AX2E2tetrahedralbent109.5°H2O 104.5°
50AX5trigonal bipyramidaltrigonal bipyramidal90°, 120°, 180°PCl5
51AX4Etrigonal bipyramidalseesaw90°, 120°, 180°SF4 101.6°, 173.1°
52AX3E2trigonal bipyramidalT-shaped90°, 180°ClF3 87.5°
53AX2E3trigonal bipyramidallinear180°XeF2
60AX6octahedraloctahedral90°, 180°SF6
61AX5Eoctahedralsquare pyramidal90°, 180°BrF5 84.8°
62AX4E2octahedralsquare planar90°, 180°XeF4

Measured angles are experimental gas-phase values from the NIST Computational Chemistry Comparison and Benchmark Database[2]; where none is listed, the angles are fixed by the molecule's symmetry. The shapes of XeF2 (linear) and XeF4 (square planar) are from OpenStax[1].

Worked example: sulfur tetrafluoride

SF4 has 6 + 4 × 7 = 34 valence electrons. Four S–F bonds use 8 and the fluorine lone pairs 24, leaving 2: one lone pair on sulfur. Four bonded atoms and one lone pair make five domains, a trigonal bipyramid. The lone pair takes an equatorial site, so the shape is a seesaw. Set 4 and 1 in the finder: the measured angles appear, 173.1° between the axial fluorines and 101.6° between the equatorial ones, both smaller than the ideal 180° and 120° because the lone pair crowds them[2].

Assumptions and limits

The finder covers one central atom with two to six electron domains, the cases VSEPR was built for. Ideal angles assume every domain is the same size; real molecules close the angles next to lone pairs and multiple bonds by a few degrees, and much more for central atoms below the second row (PH3 is 93.3°, not 107°). Molecules with two or more central atoms take a shape at each one.

Sources

  1. P. Flowers, K. Theopold, R. Langley and W. R. Robinson (2019). Chemistry 2e, §7.6 Molecular Structure and Polarity. OpenStax, Rice University. textbook
  2. R. D. Johnson III (editor) (2022). NIST Computational Chemistry Comparison and Benchmark Database (CCCBDB), NIST Standard Reference Database 101, Release 22: experimental geometries and experimental dipole moments. National Institute of Standards and Technology. database