The Shapes of Molecules and Ions

The geometric shape around an atom can be determined by considering the regions of high electron concentration around the atom. Regions of high electron concentration are the sum of bonding pairs (sigma bonds) and lone pairs of electrons and can be determined from a Lewis structure. The regions of high electron concentration are called valence-shell electron pairs. The shape is obtained by assuming the regions of high electron concentration minimize electron-electron repulsions by trying to get as far away as possible from each other and in the process adopt the specific geometries given in the following table..

If the valence-shell electron pairs are all bonding pairs of electrons (sigma bonds), then the geometry is that given above. When one or more of the valence-shell electron pairs are lone pairs of electrons the geometry is derived from that given above. 

For example, consider the methane and water molecules. The Lewis structure of methane, CH₄, shows 4 bonding pairs of electrons and no lone pairs or nonbonding pairs. Therefore, the shape of methane is tetrahedral and the bond angles are 109^o. But for water, H₂O, the Lewis structure has 2 bonding pairs of electrons and 2 nonbonding pairs of electrons or 4 regions of high electron concentration. Therefore, the shape adopted by these regions of high electron concentration is tetrahedral. However, the actual shape of H₂O is bent, with bond angles of about 109^o because we do not include the lone pairs in describing the actual shape of the molecule.  

These ideas are called the valence-shell electron-pair repulsion model, VSEPR, model.

The concepts discussed above are illustrated in the interactive molecular structure tutorial. This tutorial shows you how to determine the shape of a molecule or ion.

Go to the Molecular Structure Tutorial.