Inorganic and Applied Chemistry

Download free books at BookBooN.com

Inorganic and Applied Chemistry

Figure 2- 15: Structures for CO 2 and H 2 O

Structures for the two molecules according to the VSEPR theory. (a) Tetrahedral arrangement of the four

electron groups surrounding the oxygen atom in the water molecule. Linear arrangement of the two

electron groups surrounding the carbon atom in the carbon dioxide molecule. (b) Molecular structures.

The tetrahedral electron group arrangement in the water molecule gives a V-shaped molecule. The linear

arrangement of the electron groups in the carbon dioxide molecule gives a linear molecule.

The VSEPR theory has thus served as a tool that enabled us to explain why a carbon dioxide molecule is

linear and why a water molecule is V-shaped. The VSEPR theory is a simple and usable tool to predict

geometries of molecules when the Lewis structure is already available giving us the number of electron

groups.

It is (as mentioned earlier) the number of electron groups surrounding the central atom that determines the

arrangement and geometry around the central atom. In Example 2- L we just saw that lone pairs occupy more

space than bond electrons. For lone pairs the following rules apply:

Lone pairs occupy more space than bond electron groups

Lone pairs will be placed as far apart from other lone pairs as possible

Lone pairs will be placed as far apart from bond electron groups as possible

These guidelines can be used to predict the arrangement of bond electron groups and lone pairs relative to

each other around the central atom when the total number of electron groups is known and when the number

of lone pairs is known. In Table 2- 1 you can see how the geometry of a molecule depends on the number of

electron groups and how many of these groups that are lone pairs.

Chemical compounds