The table below contains links to pages which explain different
geometries of various molecules. These models are intended
to help you develop a mental image of the various shapes.
To view these pages you need to have RasMol and you must download
the molecules given and open them in RasMol.
To be able to predict the shape of a molecule, you must first
be able to draw a correct Lewis structure. If you are dealing
with an ion, make sure that you have the correct number of electrons
to give the indicated charge.
Once you have the Lewis structure, you can find the steric number
of the central atom. The steric number is equal to the number
of atoms attached to the central atom plus the number of lone
pairs of valence electrons on the central atom.
Note that the number of bonds on the central atom is not what matters - only the number of atoms attached to the central atom.
Finally, use the steric number to deduce the geometry. You will need to know ("memorize") the basic shape associated with each steric number (the contents of the table below). But you should not need to memorize the other shapes which arise when lone pairs are present. Those can be deduced if you can visualize the basic shape for the steric number.
| Steric Number | Orbital Geometry |
|---|---|
| 2 | Linear |
| 3 | Trigonal Planar |
| 4 | Tetrahedral |
| 5 | Trigonal Bipyramidal |
| 6 | Octahedral |