Chemistry Reference
In-Depth Information
Depending on the types and numbers of atoms involved, the molecules can
form many different shapes. The valence shell electron pair repulsion
(VSEPR) theory suggests that the shape a molecule forms is based on the
valence electrons surrounding the central atom. In this lesson, we will ex-
plore the process of predicting the shapes of molecules.
When you are dealing with a molecule that is made up of only two
atoms, HCl for example, it is easy to see that the nuclei of the atoms must
fall along a straight line. We call such a molecular shape linear. Here are
some examples.
Some Examples of Linear Molecules
F - F
H - Br
H - Cl
Diatomic Fluorine
Hydrogen Bromide
Hydrogen Chloride
Figure 4-5a
When predicting the shape of a molecule made up of three atoms, we
find that it is not as easy. CO
2
, for example, is linear, with all three of its
atoms lining up in a straight line. SO
2
, however, with almost the same for-
mula, is bent. Why the difference? The answer lies with the electrons found
in the valence shell of the central atoms in each molecule, the carbon and
the sulfur. Let's start by comparing the Lewis dot diagrams of each of these
atoms.
Comparing the Lewis Dot Diagrams of Carbon and Sulfur
C
S
Figure 4-5b
As you can see in Figure 4-5b, the sulfur atom has only two unpaired
electrons and can complete its valence shell by forming two single bonds.
The carbon, on the other hand, only has four valence electrons, so making
two single bonds won't satisfy the octet rule. To become stable when com-
bining with two oxygen atoms, carbon dioxide must make two double bonds,
sharing a total of eight valence electrons. Let's look at the Lewis dot dia-
grams for the two compounds in Figure 4-5c on page 136.
