While Lewis dot structures can tell us how the atoms in molecules are bonded to each other, they don’t tell us the shape of the molecule. In this section, we’ll discuss the methods for predicting molecular shape. The most important thing to remember when attempting to predict the shape of a molecule based on its chemical formula and the basic premises of the VSPER model is that the molecule will assume the shape that most minimizes electron pair repulsions. In attempting to minimize electron pair repulsions, two types of electron sets must be considered: electrons can exist in bonding pairs, which are involved in creating a single or multiple covalent bond, or nonbonding pairs, which are pairs of electrons that are not involved in a bond, but are localized to a single atom.
The VSPER Model—Determining Molecular Shape
| Total number of single bonds, double bonds, and lone pairs on the central atom |
Structural pair geometry |
Shape |
| 2 |
Linear |
 |
| 3 |
Trigonal planar |
 |
| 4 |
Tetrahedral |
 |
| 5 |
Trigonal bipyramidal |
 |
| 6 |
Octahedral |
 |
The above table represents a single atom with all of the electrons that would be associated with it as a result of the bonds it forms with other atoms plus its lone electron pairs. However, since atoms in a molecule can never be considered alone, the shape of the actual molecule might be different from what you’d predict based on its structural pair geometry. You use the structural pair geometry to determine the molecular geometry by following these steps:
- Draw the Lewis dot structure for the molecule and count the total number of single bonds, multiple bonds, and unpaired electrons.
- Determine the structural pair geometry for the molecule by arranging the electron pairs so that the repulsions are minimized (based on the table).
- Use the table above to determine the molecular geometry.
The table below shows all of the commonly occurring molecular geometries that are found for molecules with four or fewer bonding domains around their central atom.
|
Electron-Domain Geometry |
Bonding Domains |
Nonbonding Domains |
Molecular Geometry |
Example |
| 2 |
 |
2 |
0 |
 |
 |
| 3 |
 |
3 |
0 |
 |
 |
| 2 |
1 |
 |
 |
| 4 |
 |
4 |
0 |
 |
 |
| 3 |
1 |
 |
 |
| 2 |
2 |
 |
 |
As you can see from the table, atoms that have normal valence—meaning atoms that have no more than four structural electron pairs and obey the octet rule (and have no lone pairs)—are tetrahedral. For instance, look at methane, which is CH4:
Ammonia (NH3), which has three sigma bonds and a lone pair, however, is trigonal pyramidal:
Water (H2O) has two lone pairs and its molecular geometry is “bent,” which is also called V shaped:
So as you can see, lone pairs have more repulsive force than do shared electron pairs, and thus they force the shared pairs to squeeze more closely together.
As a final note, you may remember that we mentioned before that only elements with a principal energy level of 3 or higher can expand their valence and violate the octet rule. This is because d electrons are necessary to make possible bonding to a fifth or sixth atom. In XeF4, there are two lone pairs and four shared pairs surrounding Xe, and two possible arrangements exist:
