Molecular Geometry




The geometrical arrangements seen in nature, i.e., flowers, stones, trees, etc. can also be observed in the microscopic world. Atoms have a definite three-dimensional space arrangement relative to each other in a molecule.

The valence shell electron pair repulsion (VSPER; pronounced "vesper") model provides some useful tools for predicting molecular geometries. This model proposes that electrons are arranged around atoms in pairs such that they are kept as far away as possible. The VSPER model is based on two important principles. On the first hand it minimizes repulsion between electrons due to electrostatic interactions. On the other hand it takes into account the very important Pauli exclusion principle where each electron pair must occupy a different spatial region about an atom.

The following table will help you understand how molecular geometry can be predicted using the VSPER model. For ease of understanding we will call a bond domain the electrons that hold together two atoms.  Atoms can share two or more electrons. In any case, if two atoms share two electrons or more they will have one bond domain.  For example, the molecule ethylene, H2C=CH2, has the carbon-carbon atoms sharing four electrons but one bond domain between the two carbon atoms. 

How to use the table to predict molecular geometry.

1. Draw Lewis structures for the molecular formula given.  Note: To chose the central atom as the one with the smallest number of valence electrons or if they all have the same number of valence electrons then choose the one in the least amount. Halogens and noble gases can expand their octet.

2. Determine the number of bond domains and the number of lone pairs of electrons.

3. To predict the molecular geometry select from the table below the 3D arrangement that has the same number of bond domains and lone pairs of electrons.

 

Molecular Geometry

Example

3D Example

Bond Domains

Lone Pair of Electrons

Linear

2

0

Triangular planar

3

0

Bent (angular)

2

1

Tetrahedral

4

0

Triangular pyramidal

3

1

Bent (angular)

2

2

Triangular bipyrimidal

5

0

Seesaw

4

1

T-Shaped

3

2

Linear

2

3

Octahedral

6

0

Square pyramidal

5

1

Square planar

4

2

 

References:

Moore, John W., Stanitski, Conrad L., Chemistry: The Molecular Science, Fifth Edition, Cengage Learning, 2015, pages 286 - 295.